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This is a bug in actionlib_msgs, and you should report it here with a link to this question. It seems the output_dir has no sensible default and/or the -o option is mandatory, but it is never checked whether the user supplies this option. Also, the genaction.py call in the linked tutorial is wrong.

I could make it work with the following command (run inside package learning_actionlib:

rosrun actionlib_msgs genaction.py -o msg/ action/move_arm.action

(BTW, actions use CamelCase by convention, so MoveArm.action would be a better name.)