If this happens to you after March 1, 2025, it's probably because the Distance Matrix API is in Legacy. It's been replaced by the Routes API. You are getting this error because this feature can't be used for future usage.

If you want to use the Distance Matrix API, or if you are still get this even after switching to the Routes API, you can manually activate the Distance Matrix API by going to this link. It might also be necessary to edit the API Key API Restrictions and add the Distance Matrix API.