You cannot edit proxy/MDRS objects (see long text)

What causes this issue?

The entered class is a generic proxy type. These proxy types cannot be
processed using the Class Builder, but using the transaction SPROXY.

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?

Switch to transaction SPROXY to process the type.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.