The system could not activate the persistent class & (see long text)

What causes this issue?

You cannot activate a persistent class and another object
simultaneously for technical reasons.

System Response

The system did not activate the persistent class.

How to fix this error?

Activate the persistent class on its own.
Select this class only in the object list and choose the <ZK>Activate
</> function from the context menu.
Alternatively, activate the class in the Class Builder.

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.