Stockkeeping units & and & differ

What causes this issue?

The "receiving" material is not managed in the same
<GL:stockkeeping_unit>stockkeeping unit</> as the "issuing" material. A
transfer posting "material to material" is only possible if both
materials are managed in the same stockkeeping unit.

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?

Enter a different material or terminate processing.

Error message extract from SAP system. Copyright SAP SE.