To make a new calendar under 'On My Mac', if you are using a server calendar like iCloud or Google Calendar, you need to turn the 'calendars & reminders' syncing off for those before trying to add the calendar. You can do that in System Preferences.
If you are trying to create a new calendar on Google Calendar you need to do it on the web (https://www.google.com/calendar). Google doesn't allow you to make new calendars throught the CalDav interface Calendar uses and will throw up an error.
My error message is "The URL https://email@example.comfirstname.lastname@example.org/ encountered HTTP error 404. Make sure the URL is correct." What's this?
All apps on my Imac, Iphone, Ipod, Ipad are updated... just seems to have lost this server....
How do I know the URL is correct or not...?