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

What causes this issue?

This GTIN is already used for other materials. The national code type
for the current material is different from the national code type used
in the other materials. A synchronization of national code type 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 type, save the materials without GTIN
assignment. Call transaction <NP>/STTPEC/TRD_ITM_SER </>to assign the
GTIN, and to synchronize the national code type for all materials using
this GTIN.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.