Exception:

You can’t activate this flow because it always runs in system context. Your Salesforce admin can help with that.

Resolution:

To activate or deactivate a flow, “Manage Flow” Permission is needed.
To activate an autolaunched flow that has a trigger(Record-Triggered Flow), “View All Data” Permission is required.

So, the user trying to activate the Flow should have the needed Permission to resolve this error or exception.

I faced this issue when activating a Record-Triggered Flow though it wasn’t configured to run in System Context. “View All Data” Permission was granted via a Permission Set and issue was resolved.