I had this problem too and trawled through many answers saying check your client Id and key hash, both of which I was positive were right.
The thing that got it working for me was I opened OAuth identities in the project management console (not Firebase) at https://console.cloud.google.com/apis/credentials and added an Android Oauth client Id with the correct signature (I did not have one of these there prior for some reason). Redownload the GoogleServices json file.
I also needed to set the webClientId to the "client_type": 3 Oauth client ID. It then worked.
I had this problem too and trawled through many answers saying check your client Id and key hash, both of which I was positive were right.
The thing that got it working for me was I opened OAuth identities in the project management console (not Firebase) at https://console.cloud.google.com/apis/credentials and added an Android Oauth client Id with the correct signature (I did not have one of these there prior for some reason). Redownload the GoogleServices json file.
I also needed to set the webClientId to the "client_type": 3 Oauth client ID. It then worked.