Supplier differs from supplier & in agreement

What causes this issue?

The supplier number you entered must be the same as that of the <GL>
<GL:outline_agreement>outline agreement</>.

How to fix this error?

INCLUDE EINGABE_PRUEFEN_NA LANGUAGE EN
If the outline agreement number is correct, you can reset the supplier
number in the <GL><GL:source_list_record>source list record</>. The
supplier of the outline agreement is then copied into the source list
record.

Error message extract from SAP system. Copyright SAP SE.