You may receive “failed to log in” or “unable to authenticate” error messages if our servers are experiencing a temporary issue or your network connectivity is poor. 
Check to ensure your mobile device has a strong network connection, or wait a short while, then try logging in again. If the issue persists, try closing and reopening the Pokémon GO app:
- Android: Tap Settings > Apps > Pokémon GO > Force Stop
- iOS: Double-click the Home button or swipe up from the bar at the bottom of the screen to see your most recently used apps, find the Pokémon GO app, and then swipe up to close the app.
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Pokémon GO Game GUIDE