Material & not included in source list despite source list requirement

What causes this issue?

A source list requirement is defined for this plant. This means that
the source of supply must be entered in the source list of the
material/plant combination before you can procure this material using
this source of supply.

How to fix this error?

Check the plant you entered for the item.
Maintain the source of supply in the source list for this
material/plant combination:
Select the item.
Select <ZK>Environment -> Source list</>.
Enter the corresponding material and plant.
Maintain the data for the source of supply and then select <ZK>Save</>.
Select <ZK>Mark</> in order to get the data for the purchase order that
was created.
Exit the purchase order and restart the SAP ordering transaction.
Select <ZK>Check</>.
<ZH>Note</>
The SAP ordering transaction must be restarted for technical reasons so
that the system can read the source list entry.

Error message extract from SAP system. Copyright SAP SE.