If you are trying to delete a calendar on Google Calendar you need to do it on the web (https://www.google.com/calendar). Google doesn't allow you to delete calendars through the CalDAV interface Calendar uses and will throw up an error.
I appreciate your response. I followed your instructions and reached the web site. I noticed three calendars, all of which I need to cancel. However I did not see a delete button, for example, and I drew a blank searching help. Clearly, I have misunderstood something, for which I apologise. Additional help would be most welcome, and renewed apologies.
I think I have the same problem. I had subscribed to a CalDAV calendar in Google and I was seeing it amongst my other Google calendars in the Mac's Calendar application application as I had set up a sync between the Calendar application and my Google calendars. Now, I did not want to see that one particular calendar in my Mac's Calendar application and I tried to solve that by clicking the delete in the right-click menu for that calendar. The unwanted calendar was removed from the Calendar application but it seems like the application also assumed that I would want to delete/unsubscribe (or something) the calendar in Google too! Since Google does not allow invoking that operation from 3rd party application, the Calendar application shows an error. But the problem is, I think, that the Calendar application still keeps trying to send the delete request to Google. I haven't figured out yet, what to do.