Revaluation run not possible due to required characteristics

What causes this issue?

INCLUDE 'KKR_KLASS_NACHBEWERTUNG' OBJECT DOKU LANGUAGE E
A revaluation run like this can only be carried out if the class does
not contain any required characteristics.
The class, however, contains at least one user-defined characteristic
which is required. This means that you cannot carry out a revaluation
run. The function was cancelled.

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?

Delete all the user-defined and required characteristics from the class
or
Define all the user-defined characteristics which are not required.

Error message extract from SAP system. Copyright SAP SE.