Hi Marco,
I would at first check which purchase order is causing the error. Execute transaction SE16 -> enter the commitment line item table COOI -> search for COOI entries by entering the object number of the order (can be found on table AUFK or by executing transaction RKACSHOW). If you know exactly which purchase order is causing the error message, you can then consider further steps (set the final invoice, delivery completed flag, etc.)
BR
Marinko