Invalid roslaunch XML syntax: [Errno 2] No such file or directory: u'/opt/ros/melodic/share/gazebo_ros/launch/launch/gazebo_empty_world.launch'

edit

launch file:

<launch>

  <arg
  name="model" 
  default="$(find kct_robot)/urdf/kct_robot.xacro"
  />


    <arg name="world"
         default="$(find gazebo_plugins)/test2/worlds/empty.world"/>
<include file="$(find gazebo_ros)/launch/launch/gazebo_empty_world.launch">

    </include>

<param name="robot_description" command="$(find xacro)/xacro --inorder $(arg model)" />

 <node
    name="joint_state_publisher"
    pkg="joint_state_publisher"
    type="joint_state_publisher" />
  <node
    name="robot_state_publisher"
    pkg="robot_state_publisher"
    type="state_publisher" />

  <node
    name="spawn_model"
    pkg="gazebo_ros"
    type="spawn_model"
    args="-x 1.3644922
      -y 3.0975677
      -z 0.0
      -R 0.0
      -P 0.0
       -Y -3.14
 -urdf  

  -param robot_description -model robot_base"

    output="screen" />

</launch>