While setting up your Google OAuth client and clicking on the “Authenticate with Google” button in order to fill in your issued Google Authentication Code within the “Authentication Code” field, you may encounter the error “Error 400: invalid_request”. Below is sample display of this error:

In order to resolve this error, you will need to set your application to “Testing” and add the account that you are using to authorize as a Test User within your Google Cloud project. Below is a screenshot on this: