External manufacturer &1 has been used in &2 manufacturer part number(s)

What causes this issue?

You have changed the external manufacturer code, but MPN-MRP conversion
of material numbers is not active in your system. The system therefore
displays manufacturer parts instead of material numbers on screens and
lists. The external manufacturer might also be displayed. The external
manufacturer &V1& has been used in &V2& manufacturer part numbers in
your system, which means that these numbers cannot be correct.

System Response

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

How to fix this error?

You can save your data and execute transaction <DS:TRAN.MPN01>MPN01</>
with a new external manufacturer in order to correct the relevant
manufacturer part numbers.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.