Invalid <param> tag: Cannot load command parameter [robot_description]: no such command [['/opt/ros/noetic/share/x code example

Example: RLException: Invalid tag: Cannot load command parameter [robot_description]: no such command [['/opt/ros/noetic/share/xacro/xacro.py', '/home/megara_1/catkin_ws/src/robot_description/urdf/robot.xacro']].

In your launch file just change the param value as
<param name="robot_description" command="$(find xacro)/xacro $(arg model)" />