I am trying the package of "navigation_tutorials-indigo-devel", the package and it works well on simulation but on the robot it gives the following message:

**[ WARN] [1524062750.418983315]: No laser scan received (and thus no pose updates have been published) for 1524062750.418840 seconds. Verify that data is being published on the /scan topic. [ WARN] [1524062765.418465723]: No laser scan received (and thus no pose updates have been published) for 1524062765.418394 seconds. Verify that data is being published on the /scan topic.**