I am using the same API key I use with your classifieds addon.
but for the calendar addon, I get an error message

and the error console reports this:
js:68 You must enable Billing on the Google Cloud Project at Google Cloud Platform Learn more at Get Started with Google Maps Platform | Google Developers Places API error: BillingNotEnabledMapError
developers.google.com
but for the calendar addon, I get an error message

and the error console reports this:
js:68 You must enable Billing on the Google Cloud Project at Google Cloud Platform Learn more at Get Started with Google Maps Platform | Google Developers Places API error: BillingNotEnabledMapError

Error Messages | Maps JavaScript API | Google for Developers
