[start_demo.launch] is neither a launch file in package [learning_tf] nor is [learning_tf] a launch file name The traceback for the exception was written to the log file
i had this error after roslaunch o my cmd roslaunch learning_tf start_demo.launch
I think you should provide more explanation about the steps taken before the error happened. But anyway the error could be because you haven't sourced your workspace. You can do it manually every time an terminal is opened or you can add it into the ~/bash.src, here is explanation: setup.bash