If you have the Google Maps API Error:

And already have the Google Maps API Key:

Go to Appearance > Zoner options > Maps > Google Maps Api Key and enter your API key. 

Please refer to this post for best understanding the problem. In the post you can find the way to your API Key.

http://support.fruitfulcode.com/hc/en-us/articles/223199788-Google-Maps-API-Key