Diagnosis
You have selected the 'P.req.del.sched' (Purchase requisition withdelivery scheduling) field in Customizing for schedule line categoriesbut this is not permitted for the following reasons:
You have not activated any item categories for the purchase requisitionor
You have chosen an item category for third-party business transactions.However, delivery scheduling for purchase requisitions is onlyappropriate for processing individual purchase orders, not for third-party orders.