Reference material's classification data has not been copied

What causes this issue?

When creating the material master record, you specified a reference
material and chose the view <LS>Classification</>.

System Response

However, the classification data of the reference material could not be
copied to the current material since, for example, one or more classes
assigned to the reference material are no longer valid.

How to fix this error?

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.