Hi @Saul, the Plugin in Catalina or Big Sur did not work for me either. I don't have a solution for you either.
What exactly are you trying to set up/achieve with notifications?
I switched to the Pushover app for iOS and macOS (one-time 10,- €) some time ago. The advantage here is that you can use a KM plugin to quickly and efficiently create a notification that appears on all devices.
You can embed URL's , like Siri Shortcuts etc.. Also an icon for a better overview of the notification (for iOS).
Here is an example for my Battery Status on my iMac:
Here is an example for my ScreenFlow export to Tweetbot:
I use these notifications to show me in ScreenFlow and FCP X the completed export.
Synchronization's processes or even the completion of a backup and much more.