Can't edit .plist in /System
/System is protected by SIP (System Integrity Protection), which prevents writes made there.

It can be disabled, but you might be better off seeing if you can make modifications some other way, e.g. creating a duplicate screensaver effect that is saved to /Library.

Catalina is already doubling-down on system alterations by placing the system on a read-only volume, so any changes you make will be short-lived.