Since 2016 Google Maps has changed the Google Maps API. For this reason you have to create a API key for your domain. You need to create a free Google account before, in case you doesnt have one.
Please visit this website:
First create a new project:
Next step is to choose a name for your project and to add your domain. It is important that you add your domain fomated like the URL in our screenshot:
After this step Google Maps API website will show you the API key in a pop up window. You don’t need to insert this API key, it’s now valid for your URL / Domain, Google will check now if for your website exist a valid API key and deliver the map.