RLException: Invalid roslaunch XML syntax: no root <launch> tag

This error occurs when the roslaunch XML file being used is missing the root <launch> tag. The <launch> tag is used to indicate the start of the roslaunch file and is required for the file to be properly parsed by ROS.

To fix this error, you will need to add the <launch> tag to the beginning of your roslaunch XML file. Here is an example of what the file should look like:

<launch>
  <!-- Your launch file contents here -->
</launch>

Make sure to save the file after adding the <launch> tag, and try running the roslaunch command again.