Issues with the API key usually generates 400:Bad Request. Since you're getting 403:Forbidden, I suggest the following:
- If you're sending requests from a Server, make sure the IP address is allowed for
the API Key.
- On the same page, check Usage and Quotas tab to make sure you're not hitting any usage limits or quota.
- Check the relevant question in this
FAQ.
I hope it helps. Cheers!