You can no longer change the control indicators in the characteristic

What causes this issue?

When such a control key is used in the characteristic, you can no
longer change the control indicators. The control key has marked the
control indicators as "fixed".

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?

Error message extract from SAP system. Copyright SAP SE.