PurchReq. delivery scheduling is not permitted for purchasing item cat. &

What causes this issue?

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

Error message extract from SAP system. Copyright SAP SE.