I read in quite a few places that many other people were having the same problem, there seems to be a problem on how they handle API usage restrictions for that API. If I create brand new credentials with no restrictions on it, it works perfectly. However, if I try to put on some restrictions on it (which you must do) , it does throw the error. I’ve tried with the exact same restrictions that I have for Maps API and doesn’t work, so seems to be a problem at their end …
I’ve ended up going with Deepl API, working pretty smoothly