No class for specialization type &1 implemented

What causes this issue?

You want to specialize a material to specialization category &v1&.
However, no class has been entered that implements the necessary
interface.

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?

Maintain the relevant class in the transaction STCU.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.