SAP Message 88619 - Document type &1 is not a main document type

Diagnosis
Only main document types of category E, P, J, S, T, or U can be settled.
The document type entered is either not a main document type or belongsto the wrong category.

System Response
This document type can not be settled.

Procedure
Choose another document type or
Flag the document type as a main document type in document typecontrol in the recycling partner dialog.