Not to dredge up an old thread, but I'm running into this as well (2014 MB Pro on BigSur). My family all runs on Google calendar. I use mostly stock Apple apps with Google incorporated (Calendar & Mail), but this is extremely frustrating. Everything is fine on my iPhone, but my Macbook does not play nice. Everything I read on the Apple support community forums seem to believe this is an Apple problem. As for now I've just relegated to Google calendar in the browser which is a real pain. I like to drag emails into the calendar to create events, but this error message prevents you from doing anything in the calendar.