Supplying plant entered incompatible with supplier in agreement

What causes this issue?

You have entered a <GL:issuing_plant>supplying plant</> and an
<GL:outline_agreement>outline agreement</>. Whereas the outline
agreement specifies external procurement, entry of a supplying plant
presupposes internal procurement.

How to fix this error?

INCLUDE EINGABE_PRUEFEN_NA LANGUAGE EN

Error message extract from SAP system. Copyright SAP SE.