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RLException: Invalid <param> tag: Cannot load command parameter [robot_description]: no such command [['/opt/ros/noetic/share/xacro/xacro.py', '/home/b/rosstudy/rosws/src/learning_ros_noetic/Part_2/mobot_urdf/urdf/mobot_w_arm_and_jnt_pub.xacro']]. 

Param xml is <param name="robot_description" command="$(find xacro)/xacro.py '$(find mobot_urdf)/urdf/mobot_w_arm_and_jnt_pub.xacro'"/>
The traceback for the exception was written to the log file