You are unable to add attachments or make any changes like accept deviation in a purchase order created with account assignment as Sales Order.
SAP Business ByDesign
When you try attach the documents in the purchase order and save the document, it creates a fresh notification in financials. In this case, sales order is in completed status. With the completed sales orders, financials cannot process the new notification at all.
When you try to accept deviation then system tries to make changes in PO and also in respective sales order but since Sales order is completed, system does not allow such changes.
So, system displays this error message when you do any changes in the purchase order.
System is working as designed.
Unable to accept deviation, cannot add attachments , KBA , AP-PRO-PPR , Project Purchase Request , AP-PRP , Purchase Request Processing , How To