On Backstage, after following all the steps on https://backstage.io/docs/auth/google/provider, I get the message "The Google provider is not configured to support sign-in" after going through the Google SSO interface.
If I understood correctly, I have to setup Backstage to enable/allow Google provider to sign-in users. But I'm lost on how to do this.
How to configure Google provider to support sign-in on Backstage?
It seems you missed the backend app's auth plugin configuration as mentioned in the docs
You should create a resolver function to map the google user's identity to a backstage user's identity or, optionally, skip the catalog's user lookup ( there are caveats to this approach ).
The following documentation explains about the resolver function and user identities: Sign-in Identities and Resolvers
The following code is an example of how you would achieve a google resolver WITHOUT mapping to a backstage user identity and I recommend using it for testing purposes only and, as previosly mentioned, there are caveats to this approach. I strongly recommend understanding the documentation and the power mapping the external user identity to a backstage user identity unleashes.