Some Pokemon GO users running older versions of the popular mobile title may be unable to properly access its contents. If you’ve received messages such as “Unable to Authenticate,” developer Niantic has confirmed that a likely cause of this is due to you using an out-of-date version of the game. This issue can be resolved by simply updating the app in the App Store on iOS or Google Play on Android devices.
Pokemon GO users may encounter issues when using old versions of the game
Pokémon Blog 
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