What causes this issue?

You tried to activate or deactivate a feature toggle. But this is
currently not possible because an upgrade is running in parallel.

System Response

The activation status of the feature toggle was not changed.

How to fix this error?

Wait until the upgrade finished and repeat your activation afterwards.

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.