Enter a valid material class

What causes this issue?

The material class you have chosen, &v1&, using grouping key &v2&, does
not exist.

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 class or choose a different grouping key.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.