This error is related to the google maps API key that you have entered in the Settings tab on the Transporters plugin config page.
If you can share the link to your website then I can help identify the exact issue but 3 common things could cause this.
1) Click https://console.developers.google.com/apis/dashboard Then click enable API and ensure that the following are all active.
Google Places API Web Service
Google Maps Geocoding API
Google Maps Directions API
Google Maps JavaScript API
2) Click https://console.developers.google.com/apis/credentials
Find the API key entered in the box on the plugin settings and make sure it matches exactly .
3) Click https://console.developers.google.com/apis/credentials
Click the key and then under Key restriction choose “HTTP referrers (web sites)”
In the list of sites to accept make sure the following is present.
*yourwebsitename.com/*
[Replace “yourwebsitename.com” with your real website]