It may happens for one of the following reason:
- 1. Make sure to have both API and Application restriction set to None or restrict them to your domain name properly from Credentials->API Keys->Your API->Application restrictions
- 3. Also, from the middle of 2018, Google requires you to add a Billing account. Although, for 99% cases you won’t actually have to pay anything as the free usage limit is quite high for almost any use cases.