Procurement w/o material from supplier with plant assignment not defined

What causes this issue?

You are entering an ordering transaction involving a supplier to whom a
plant has been assigned. Normally, materials would be supplied by one
plant to another in such a transaction.
This process involves a delivery and billing document in the SD system.
The PO for this appears directly in the delivery due list of the issuing
plant (supplier) and therefore does not need to be transmitted.
However, items without a material master record cannot appear in the
delivery due list.

How to fix this error?

Items without a material master record can only be ordered from
suppliers without a plant assignment.

Error message extract from SAP system. Copyright SAP SE.