What causes this issue?

The input composition of specification &v1& contains a mix of material
categories that are not allowed: you can use basic materials or
surfaces, or you can use other material categories, but don't mix them.

System Response

The migration will skip this input composition and continue with the
next specification.

How to fix this error?

Correct the input composition.

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.