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This error usually means your ros-master-node has died. This application is named roscore, and ps -ef | grep roscore should return a line something like this:

user1 15240 23167  0  2021 pts/6    00:04:46 /usr/bin/python3 /opt/ros/noetic/bin/roscore -p 11311

If you don't see this process listed, to recover you must stop all your ros nodes, then run roslaunch again.

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