Diagnosis
This can be caused by one of the following:
End of number range
WM reference locked
Document already included in a WM reference
Category of document is incorrect
Unforeseen error situation
Unforeseen error situation
In case 1 the number range must be set up again.
Cases 2 - 6 are probably due to an error in a ABAP/4 program.

System Response
It is not possible to create the picking waves.

Procedure
For case 1, contact the person responsible for Customizing or yoursystem administrator.
In all other cases, inform your system administrator.