Validation of Google User Token in the back end

Question

I’m facing a problem and I’ve already lost hours on forums and on google certification, but I can’t find the solution. I have the following scenario:

1. A front-end that is from a third company logs in through the google service, an application in Firebase, authenticating the user and making a request for my back-end, where it sends me the accessToken (JWT Token).

2. In the back-end I pick up the accessToken and need to fetch user information. I am using the official google library to . Net (Google.Apis.Oauth2.v2). However, when the code below is exceeded, I get the error from Badrequest - Error Response is null.

    try
    {                
        Oauth2Service service = new Oauth2Service(new BaseClientService.Initializer());
        Oauth2Service.TokeninfoRequest request = service.Tokeninfo();
        request.AccessToken = token;
   
        Tokeninfo info = await request.ExecuteAsync();
        return info.UserId;
    }
    catch (Exception ex)
    {
        Debug.WriteLine(ex.Message);
    }
   
    return string.Empty;
   

When executing the above code I get the following error:

If someone has already been through the same problem, or has an accessToken verification solution in the back end and can help, I would appreciate it. Hugs.