Diagnosis
The system does not have a selection category that selects by documentnumber (field name OPBEL). However, returns can only be uniquelyallocated using a document number.

System Response
The system cannot make a selection and therefore cannot process thereturns lot.

Procedure for System Administration
Check your system settings.
Create a new selection category with the field name "OPBEL".
To do this, choose Maintain entries.