You've probably created a node set at some time and then changed the model so that the geometry no longer exists. Go back into CAE and edit/delete all your node sets and surfaces etc. to make sure they are still valid.  

if you r defining reference point and want to apply force or moments etc on that point then it will show error of above kind . to get rid of the error go to interaction ->create constrain->coupling

and couple the point to the surface .

Thanks