Contract supplier &1 is different from purchase order supplier &2

What causes this issue?

The contract that you are referring to has the supplier number &V1&, but
the supplier for the purchase order has the number &V2&. Both numbers
must be identical.

System Response

INCLUDE ISR_STOP

How to fix this error?

INCLUDE ISR_CHECK

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.