The inbound delivery still has items that do not refer to a PO

What causes this issue?

Delivery type &v1& does not allow you to save an inbound delivery when
one or more items do not have a reference to a purchase order.

System Response

The system cannot save the inbound delivery.

How to fix this error?

Either
complete the items that do not refer to a purchase order
use a different delivery type that allows incomplete inbound deliveries
change delivery type &v1& accordingly

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.