I'm relatively new to ROS. I started with raw URDF exported from solidworks. Then created moveitconfig package by moveitsetup_assistant. Here are the related files: URDF file:

<robot name="prochesta_arm">
    <material name="white">
        <color rgba="1.0 1.0 1.0 1.0"/>
    </material>
    <material name="grey">
        <color rgba="0.5 0.5 0.5 1.0"/>
    </material>
    <material name="grey_2">
        <color rgba="0.792156862745098 0.819607843137255 0.933333333333333 1" />
    </material>
    <material name="pink">
        <color rgba="1.0 0.5 0.5 1.0" />
    </material>
    <material name="blue">
        <color rgba="0.0 0.4 1.0 1.0" />
    </material>
    <material name="green">
        <color rgba="0.0 1.0 0.5 1.0" />
    </material>
    <link name="base_bar">
        <!--inertial>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <mass value="0.392347184025621" />
            <inertia ixx="0.0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0" />
        </inertial-->
        <visual>
            <origin xyz="0.0 0.0 0.05" rpy="0.0 0.0 0.0"/>
            <geometry>
                <box size="0.01 0.01 0.1"/>
            </geometry>
            <material name="grey"/>
        </visual>
        <collision>
            <origin xyz="0.0 0.0 0.05" rpy="0.0 0.0 0.0"/>
            <geometry>
                <box size="0.01 0.01 0.1"/>
            </geometry>
        </collision>
    </link>
    <joint name="base_plane__ArmBase" type="fixed">
        <origin rpy="0.0 0.0 0.0" xyz="0.0546412131471822 0.0 0.11" />
        <parent link="base_bar" />
        <child link="ArmBase" />
        <axis xyz="0.0 0.0 0.0" />
        <limit lower="0.0" upper="0.0" effort="1000.0" velocity="50.0"/>
    </joint>
    <joint name="base_bar__base_plane" type="fixed">
        <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>
        <parent link="base_plane"/>
        <child link="base_bar"/>
        <axis xyz="0.0 0.0 0.0"/>
        <limit lower="0.0" upper="0.0" effort="0.0" velocity="0.0"/>
    </joint>
    <link name="base_plane">
        <!--inertial>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <mass value="0.392347184025621" />
            <inertia ixx="0.0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0" />
        </inertial-->
        <visual name="">
            <origin xyz="0.0 0.0 0.01" rpy="0.0 0.0 0.0"/>
            <geometry>
                <box size="1.5 1.5 0.02"/>
            </geometry>
            <material name="grey"/>
        </visual>
        <collision>
            <origin xyz="0.0 0.0 0.01" rpy="0.0 0.0 0.0"/>
            <geometry>
                <box size="1.5 1.5 0.02"/>
            </geometry>
        </collision>
    </link>
    <link name="ArmBase">
        <inertial>
            <origin rpy="0 0 0" xyz="-0.0546412131471822 0.0142569733044021 0.0105265889786102" />
            <mass value="0.392347184025621" />
            <inertia ixx="0.00037313359562078" ixy="-3.88508187746182E-07" ixz="-9.7227823468437E-06" iyy="0.000612254064917396" iyz="6.685567634577E-07" izz="0.000292429195752973" />
        </inertial>
        <visual>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/ArmBase.STL" />
            </geometry>
            <material name="white"/>
        </visual>
        <collision>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/ArmBase.STL" />
            </geometry>
        </collision>
    </link>
    <link name="BaseShaft">
        <inertial>
            <origin rpy="0 0 0" xyz="-0.00124611489784311 -0.108184816547343 -1.18094289902615E-09" />
            <mass value="0.172603193441375" />
            <inertia ixx="0.000449963539893855" ixy="2.29673976828531E-06" ixz="4.03285265163475E-09" iyy="0.000436230277248471" iyz="-5.17320253939005E-12" izz="0.000511313152396509" />
        </inertial>
        <visual>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/BaseShaft.STL" />
            </geometry>
            <material name="pink"/>
        </visual>
        <collision>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/BaseShaft.STL" />
            </geometry>
        </collision>
    </link>
    <joint name="ArmBase__BaseShaft" type="revolute">
        <origin rpy="-1.5708 0 -1.5708" xyz="-0.075617 0.021609 0" />
        <parent link="ArmBase" />
        <child link="BaseShaft" />
        <axis xyz="0 1 0" />
        <limit effort="1000" lower="-3.1416" upper="3.1416" velocity="50" />
        <dynamics damping="10.0" friction="10.0"/>
    </joint>
    <link name="Shoulder">
        <inertial>
            <origin rpy="0 0 0" xyz="0.00251674474561527 0.170121527752926 0.00159696012847385" />
            <mass value="0.100791466963889" />
            <inertia ixx="0.000469966041530564" ixy="-5.89764046954074E-06" ixz="5.65207828543954E-09" iyy="7.13384967018999E-05" iyz="-3.44218965703515E-07" izz="0.000512104689605307" />
        </inertial>
        <visual>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/Shoulder.STL" />
            </geometry>
            <material name="white"/>
        </visual>
        <collision>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/Shoulder.STL" />
            </geometry>
        </collision>
    </link>
    <joint name="BaseShaft__Shoulder" type="revolute">
        <origin rpy="1.5535 1.556 -1.5873" xyz="-0.02 -0.1456 0" />
        <parent link="BaseShaft" />
        <child link="Shoulder" />
        <axis xyz="-0.99989 0.01479 0.00025546" />
        <!-- limits have been calculated manually-->
        <limit effort="1000" lower="-1" upper=".5" velocity="50" />
        <dynamics damping="10.0" friction="10.0"/>
    </joint>
    <link name="Elbow">
        <inertial>
            <origin rpy="0 0 0" xyz="-4.98394797654456E-11 -3.62130138409911E-05 -0.17234820422493" />
            <mass value="0.155859341630008" />
            <inertia ixx="0.000268369583946352" ixy="2.061494260678E-10" ixz="-6.45772822678737E-11" iyy="0.000298530192977532" iyz="-4.44744351270454E-05" izz="0.000181542359930575" />
        </inertial>
        <visual>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/Elbow.STL" />
            </geometry>
            <material name="pink"/>
        </visual>
        <collision>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/Elbow.STL" />
            </geometry>
        </collision>
    </link>
    <joint name="Shoulder__Elbow" type="revolute">
        <origin rpy="-2.8163 -0.00025546 3.1268" xyz="0.0051765 0.34996 0.00027318" />
        <parent link="Shoulder" />
        <child link="Elbow" />
        <axis xyz="1 0 0" />
        <!-- limits have been calculated manually-->
        <limit effort="1000" lower="-0.5" upper=".7837" velocity="50" />
        <dynamics damping="10.0" friction="10.0"/>
    </joint>
    <link name="Yaw">
        <inertial>
            <origin rpy="0 0 0" xyz="-0.00438644516368353 0.0559409791205142 -0.0570850114857289" />
            <mass value="0.296477336396768" />
            <inertia ixx="0.000492376543215998" ixy="-1.09052812794105E-05" ixz="-2.66243025248788E-05" iyy="0.000302345188534722" iyz="-2.34599297023637E-05" izz="0.000282182542169169" />
        </inertial>
        <visual>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/Yaw.STL" />
            </geometry>
            <material name="blue"/>
        </visual>
        <collision>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/Yaw.STL" />
            </geometry>
        </collision>
    </link>
    <joint name="Elbow__Yaw" type="revolute">
        <origin rpy="-0.0016492 -0.098555 0.033437" xyz="0 -0.098331 -0.29083" />
        <parent link="Elbow" />
        <child link="Yaw" />
        <axis xyz="0 0.94732 -0.3203" />
        <!-- limits have been calculated manually-->
        <limit effort="1000" lower="-1.5708" upper="1.5708" velocity="50" />
        <dynamics damping="10.0" friction="10.0"/>
    </joint>
    <link name="Pitch">
        <inertial>
            <origin rpy="0 0 0" xyz="0.00193637684331768 -0.00113094288383903 -0.00473550556070457" />
            <mass value="0.245672351084059" />
            <inertia ixx="0.000112445414870033" ixy="3.7801113676169E-06" ixz="1.13310562352918E-05" iyy="0.000222075669730552" iyz="8.19497975087401E-07" izz="0.000225909542391733" />
        </inertial>
        <visual>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/Pitch.STL" />
            </geometry>
            <material name="pink"/>
        </visual>
        <collision>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/Pitch.STL" />
            </geometry>
        </collision>
    </link>
    <joint name="Yaw__Pitch" type="revolute">
        <origin rpy="-0.095533 0.0036424 0.0092858" xyz="-0.013295 0.024778 -0.14277" />
        <parent link="Yaw" />
        <child link="Pitch" />
        <axis xyz="0.99459 -0.033268 -0.098395" />
        <!-- limits have been calculated manually-->
        <limit effort="1000" lower="-1.5708" upper="1.5708" velocity="50" />
        <dynamics damping="10.0" friction="10.0"/>
    </joint>
    <link name="Roll">
        <inertial>
            <origin rpy="0 0 0" xyz="0.013346395763443 0.026611833443925 0.0965457711475227" />
            <mass value="0.219051002803153" />
            <inertia ixx="0.000155851657200548" ixy="-1.61881929035119E-05" ixz="1.50471506253359E-05" iyy="0.000101493190927247" iyz="4.64669877102981E-05" izz="0.000174715266607372" />
        </inertial>
        <visual>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/Roll.STL" />
            </geometry>
            <material name="white"/>
        </visual>
        <collision>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/Roll.STL" />
            </geometry>
        </collision>
    </link>
    <joint name="Pitch__Roll" type="revolute">
        <origin rpy="-2.9699 0.21413 -0.25831" xyz="0 0 0" />
        <parent link="Pitch" />
        <child link="Roll" />
        <axis xyz="0.16462 0.40474 0.89949" />
        <!-- limits have been calculated manually-->
        <limit effort="1000" lower="-3.1416" upper="3.1416" velocity="50" />
        <dynamics damping="10.0" friction="10.0"/>
    </joint>
    <link name="LeftClaw">
        <inertial>
            <origin rpy="0 0 0" xyz="0.0160548042850929 0.363052679901909 -0.277299510990579" />
            <mass value="0.0710508103277403" />
            <inertia ixx="6.33292286188025E-05" ixy="-7.37244945176046E-06" ixz="-1.6248611953797E-05" iyy="5.934165926219E-05" iyz="-1.6824179025353E-05" izz="2.45890867032075E-05" />
        </inertial>
        <visual>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/LeftClaw.STL" />
            </geometry>
            <material name="green"/>
        </visual>
        <collision>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/LeftClaw.STL" />
            </geometry>
        </collision>
    </link>
    <link name="RightClaw">
        <inertial>
            <origin rpy="0 0 0" xyz="0.0160548042897368 0.363052680008611 -0.277299511621476" />
            <mass value="0.0710508080215541" />
            <inertia ixx="6.33292264052498E-05" ixy="-7.37244969866602E-06" ixz="-1.62486114309369E-05" iyy="5.9341657110105E-05" iyz="-1.68241784848982E-05" izz="2.45890872002616E-05" />
        </inertial>
        <visual>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/RightClaw.STL" />
            </geometry>
            <material name="blue"/>
        </visual>
        <collision>
            <origin rpy="0 0 0" xyz="0 0 0" />
            <geometry>
                <mesh filename="package://prochesta_arm/meshes/RightClaw.STL" />
            </geometry>
        </collision>
    </link>
    <joint name="Joint_LeftClaw" type="prismatic">
        <origin rpy="0 0 0" xyz="-0.0088606 -0.28846 0.43314" />
        <parent link="Roll" />
        <child link="LeftClaw" />
        <axis xyz="-0.96885 0.23743 0.070474" />
        <limit effort="1000" lower="0" upper="0.021" velocity="50" />
        <dynamics damping="10.0" friction="10.0"/>
    </joint>
    <joint name="Joint_RightClaw" type="prismatic">
        <origin rpy="0.86688 -0.30065 3.0016" xyz="0.099182 0.50791 0.05503" />
        <parent link="Roll" />
        <child link="RightClaw" />
        <axis xyz="0.96885 -0.23743 -0.070474" />
        <limit effort="1000" lower="-0.021" upper="0" velocity="50" />
        <dynamics damping="10.0" friction="10.0"/>
    </joint>


    <transmission name="trans_ArmBase__BaseShaft">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="ArmBase__BaseShaft">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
        </joint>
        <actuator name="ArmBase__BaseShaft_motor">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    <transmission name="trans_BaseShaft__Shoulder">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="BaseShaft__Shoulder">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
        </joint>
        <actuator name="BaseShaft__Shoulder_motor">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    <transmission name="trans_Shoulder__Elbow">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="Shoulder__Elbow">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
        </joint>
        <actuator name="Shoulder__Elbow_motor">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    <transmission name="trans_Elbow__Yaw">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="Elbow__Yaw">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
        </joint>
        <actuator name="Elbow__Yaw_motor">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    <transmission name="trans_Yaw__Pitch">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="Yaw__Pitch">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
        </joint>
        <actuator name="Yaw__Pitch_motor">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    <transmission name="trans_Pitch__Roll">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="Pitch__Roll">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
        </joint>
        <actuator name="Pitch__Roll_motor">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    <transmission name="trans_Joint_LeftClaw">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="Joint_LeftClaw">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
        </joint>
        <actuator name="Joint_LeftClaw_motor">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    <transmission name="trans_Joint_RightClaw">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="Joint_RightClaw">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
        </joint>
        <actuator name="Joint_RightClaw_motor">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    <transmission name="trans_base_plane__ArmBase">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="base_plane__ArmBase">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
        </joint>
        <actuator name="base_plane__ArmBase_motor">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    <transmission name="trans_base_bar__base_plane">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="base_bar__base_plane">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
        </joint>
        <actuator name="base_bar__base_plane_motor">
            <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    <gazebo>
        <plugin filename="libgazebo_ros_control.so" name="gazebo_ros_control">
            <robotNamespace>/</robotNamespace>
            <legacyModeNS>true</legacyModeNS>
        </plugin>
        <!--plugin filename="libgazebo_ros_joint_state_publisher.so" name="joint_state_publisher">
            <robotNamespace>/</robotNamespace>
            <jointName>
                ArmBase__BaseShaft, 
                BaseShaft__Shoulder, 
                Elbow__Yaw, 
                Pitch__Roll,
                Shoulder__Elbow,
                Yaw__Pitch,
                Joint_LeftClaw, 
                Joint_RightClaw
            </jointName>
        </plugin-->
    </gazebo>
</robot>

gazebo.launch file: <?xml version="1.0"?>

<!-- startup simulated world -->
<include file="$(find gazebo_ros)/launch/empty_world.launch">
  <arg name="world_name" default="worlds/empty.world"/>
  <arg name="paused" value="true"/>
  <arg name="gui" value="$(arg gazebo_gui)"/>
</include>

<!-- send robot urdf to param server -->
<param name="robot_description" command="xacro  '$(find mybot)/urdf/mybot.urdf.xacro'" />

<!-- unpause only after loading robot model -->
<arg name="unpause" value="$(eval '' if arg('paused') else '-unpause')" />
<!-- push robot_description to factory and spawn robot in gazebo at the origin, change x,y,z arguments to spawn in a different position -->
<arg name="world_pose" value="-x 0 -y 0 -z 0" />
<node name="spawn_gazebo_model" pkg="gazebo_ros" type="spawn_model" args="-urdf -param robot_description -model robot $(arg unpause) $(arg world_pose) $(arg initial_joint_positions)"
  respawn="false" output="screen" />

<!-- Load joint controller parameters for Gazebo -->
<rosparam file="$(find mybot_moveit_config)/config/gazebo_controllers.yaml" />
<!-- Spawn Gazebo ROS controllers -->
<node name="gazebo_controller_spawner" pkg="controller_manager" type="spawner" respawn="false" output="screen" args="joint_state_controller" />
<!-- Load ROS controllers -->
<include file="$(dirname)/ros_controllers.launch"/>

After launch the error I get:

[ERROR] [1681988267.705543000]: This robot has a joint named "base_plane__ArmBase" which is not in the gazebo model.
[FATAL] [1681988267.705556000]: Could not initialize robot simulation interface

...

[WARN] [1681988296.560628, 28.000000]: Controller Spawner couldn't find the expected controller_manager ROS interface.
[WARN] [1681988296.561593, 28.001000]: Controller Spawner couldn't find the expected controller_manager ROS interface.

The gazeboroscontroller plugin doesn't load. I have a similar arm which I have set up exactly the same way and that works expectedly whereas this one just falls down in the gazebo.

I have found similar problems in this forum and over the internet, but none of them works.