Not possible to assign a source to all items

What causes this issue?

The system was not able to determine a <GL><GL:source_of_supply>source
of supply</> for all items.

How to fix this error?

When creating a requisition:
You can save the PO as usual - the purchase requisition will be
processed by purchasing without assignment. You can also enter a source
manually in the item detail data (under "sources of supply").
When creating a PO with " supplier unknown":
If you cannot currently assign a source to the items, you have the
option of storing the items as a purchase requisition (via F13).

Error message extract from SAP system. Copyright SAP SE.