What's the difference between 'Published app' and 'Testing' in the Google Cloud Console, and how does it relate to this error?

A 'Published app' is available to all Google users who meet the criteria defined in your OAuth consent screen. 'Testing' allows you to restrict access to a specific list of users (test users). If your app is in 'Testing' mode and a user *not* on the test users list tries to access it, they'll receive the 'app_not_configured_for_user' error. You need to publish the app or add the user to the test list.