service layer:DraftsService_SaveDraftToDocument fails

Hi Expert

I use the service layer DraftsService_SaveDraftToDocument interface,Save documents Object Code is Purchase Returns drafts(oOrders,oCreditNotes,oPurchaseCreditNotes) save official documents and SAP returns internal errors.

But if it is unsourced draft save official documents, return successful and SAP returns successful.

Thank you for your Help.