This is usually a sign that NO APIs we're enabled inside the Google Cloud Platform.
Also please see from video tutorials and PDF guide that there are two APIs you must enable.
If you have CitationOptimiser, please follow this PDF guide too. There is another service you must enable on Google API.
(THIS IS ONLY FOR CitationOptimiser Module)
Also, when you copy and paste the API key to paste it inside Mapify360, make sure that there are no empty spaces before or after.