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roslaunch
tries to start the roscore nodes by loading the roscore launch file from your ROS distribution, for instance /opt/ros/melodic/etc/ros/roscore.xml
. This launch file instructs roslaunch
to execute rosversion roslaunch
.
If rosversion roslaunch
fails for some reason, this error will occur. You should try to run this command separately to see what error is displayed.
A reason why it might fail is if your ROS environment variables are not correct. You should be using source /opt/ros/melodic/setup.bash
or similar (see this tutorial), plus a separate Catkin workspace file, to populate variables.
I encountered this issue myself because I was overriding ROS_PACKAGE_PATH
incorrectly. Once I fixed it, the error went away.