URDF model does not appear in Gazebo: no errors

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I am using ROS noetic on Ubuntu 20.04. Gazebo version 11.9.0. PC specifications: Intel core I7-7700H, 4GB Nvidia 940MX, 16 GB DDR4 RAM

This is my main file:

<?xml version="1.0"?>
<robot xmlns:xacro="http://ros.org/wiki/xarco" name="robot_base">
  <xacro:include filename="$(find robot_description)/urdf/robot_base_essentials.xacro"/>
  <xacro:include filename="$(find robot_description)/urdf/gazebo_essentials_base.xacro" />

  <link name="base_link">
    <visual>
      <origin xyz="0 0 0" rpy="1.5707963267949 0 3.14"/>
      <geometry>
        <mesh filename="file:///robot_description/meshes/robot_base.stl"/>
      </geometry>
      <material name="">
        <color rgba="0.79216 0.81961 0.93333 1"/>
      </material>
    </visual>
    <collsion>
      <origin xyz="0 0 0" rpy="1.5707963267949 0 3.14"/>
      <geometry>
        <mesh filename="file:///robot_description/meshes/robot_base.stl"/>
      </geometry>
    </collsion>
    <inertial> 
      <origin xyz="0.0030946 4.78250032638821E-11 0.053305" rpy="0 0 0"/>
      <mass value="47.873"/>
      <inertia ixx="0.774276574699151" ixy="-1.03781944357671E-10" ixz="0.00763014265820928" iyy="1.64933255189991" iyz="1.09578155845563E-12" izz="2.1239326987473" />
    </inertial>
  </link>
  <xacro:robot_wheel prefix="front_left"/>
  <xacro:robot_wheel prefix="front_right"/>
  <xacro:robot_wheel prefix="rear_left"/>
  <xacro:robot_wheel prefix="rear_right"/>

  <xacro:wheel_joint prefix="front_left" origin="0.220 0.250 0"/>
  <xacro:wheel_joint prefix="front_right" origin="0.220 -0.250 0"/>
  <xacro:wheel_joint prefix="rear_left" origin="-0.220 0.250 0"/>
  <xacro:wheel_joint prefix="rear_right" origin="-0.220 -0.250 0"/>

  <xacro:base_transmission prefix="front_left"/>
  <xacro:base_transmission prefix="front_right"/>
  <xacro:base_transmission prefix="rear_left"/>
  <xacro:base_transmission prefix="rear_right"/>

  <xacro:wheel_friction prefix="front_left"/>
  <xacro:wheel_friction prefix="front_right"/>
  <xacro:wheel_friction prefix="rear_left"/>
  <xacro:wheel_friction prefix="rear_right"/>

</robot>

This is my xacro file:

<?xml version="1.0"?>
<robot xmlns:xacro="http://ros.org/wiki/xacro" name="robot_essentials">
  <xacro:macro name="robot_wheel" params="prefix">
    <link name="${prefix}_wheel">
      <visual>
        <origin xyz="0 0 0" rpy="1.5707963267949 0 0"/>
        <geometry>
          <mesh filename="file:///robot_description/meshes/wheel.stl"/>
        </geometry>
        <material name="">
          <color rgba="0.79216 0.81961 0.93333 1"/>
        </material>
      </visual>
      <collision>
        <origin xyz= "0 0 0" rpy="1.5707963267949"/>
        <geometry>
          <mesh filename="file:///robot_description/meshes/wheel.stl"/>
        </geometry>
      </collision>
      <inertial>
        <origin xyz="-4.1867E-18 0.0068085 -1.65658661799998E-18" rpy="0 0 0"/>
        <mass value="2.6578"/>
        <inertia ixx="0.00856502765719703" ixy="1.5074118157338E-19" ixz="-4.78150098725052E-19" iyy="0.013670640432096" iyz="-2.68136447099727E-19" izz="0.00856502765719703" />
      </inertial>
    </link>
  </xacro:macro>
  <xacro:macro name="wheel_joint" params="prefix origin">
    <joint name="${prefix}_wheel_joint" type="continuous">
      <axis xyz="0 1 0"/>
      <parent link="base_link"/>
      <child link="${prefix}_wheel"/>
      <origin rpy="0 0 0" xyz="${origin}"/>
    </joint>
  </xacro:macro>
  <xacro:macro name="base_transmission" params="prefix">
    <transmission name="${prefix}_wheel_trans" type="SimpleTransmission">
      <type>transmission_interface/SimpleTransmission</type>
      <actuator name="${prefix}_wheel_motor">
        <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
        <mechanicalReduction>1</mechanicalReduction>
      < ...