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
						
					
					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
 developers.google.com
						
					
					developers.google.com
				 
 
		
 
						
					 
 
		