Ros tutorial problem - roscd

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I create a ros package, while I us roscd that cannot find it.

sunli@sunli-ThinkPad-T410:~/catkin_ws/src$ roscd begin_tutorials roscd: No such package/stack 'begin_tutorials'

Then, I use rosws set,

sunli@sunli-ThinkPad-T410:~/catkin_ws/src$ rosws set beginner_tutorials ERROR in config: Command requires a target workspace.

This problem often occurs and sometime I can fix it but I dont know what's the exact reason?

Thanks a lot!