Source of supply blocked (according to source list)

What causes this issue?

No purchase orders can be created for this <GL:source_of_supply>source
</> because the source is blocked in the <GL:source_list>source list</>
for this material.

How to fix this error?

Try ordering from another source.
If the material has to be procured from this source of supply, you will
have to "unblock" it in the source list:
<DS:TRAN.ME01>Perform function</>
If the source list was maintained via an outline agreement item, you
must unblock the source list entry via the outline agreement.

Error message extract from SAP system. Copyright SAP SE.