What causes this issue?

A client specific activation or deactivation of a Feature Toggle was
requested. But such an action is not possible because the Feature Toggle
is currently active system wide.

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?

First you can deactivate the Feature Toggle system wide and in a second
step add your required client restrictions.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.