GTIN &1 is already used in material &2. National code mismatch. &3/&4

What causes this issue?

This GTIN is already used for other materials. The national code for the
current material is different from the national code used in the other
materials. A synchronization of national code between all materials is
not possible with the transactions MM01/MM02.

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

System Response

How to fix this error?

To synchronize the national code, save the materials without GTIN
assignment. Call transaction <NP>/STTPEC/TRD_ITM_SER </>to assign the
GTIN, and to synchronize the national code for all materials using this
GTIN.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.