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This error indicates that there is a syntax error in your launch file: Invalid roslaunch XML syntax: mismatched tag: line 4, column 6

"mismatched tag" in XML means that you have a close tag (starts with </) that the XML parser wasn't expecting. In this case, you've closed the node tag twice: once by using a self-closing tag (<node/>) and again with an explicit close tag on line 4, where the parser pointed out your error (</node>).

You should change your launch file so that you either use matching open and close tags ( <node> and </node> ) or use the self-closing tag ( <node/> )

I suggest

<launch>
    <node pkg="exercise_21" type="move_robot.py" name="move_robot_node" output="screen" />
</launch>