I have a Google Fi physical SIM card inserted in the phone and activated it using the Google Fi app (in the owner profile with sandboxed Google Play services installed). I cannot sign in to my Google Fi account in Google Messages app (profile picture -> Messages settings -> Advanced -> Google Fi Wireless settings -> Sign in to your Google Fi account). After selecting my account, it prompts "To continue signing in, select the Google Fi account used on this device". I have given every permission that Google Fi app, Google Messages app, and Google Play services ask.