Please try completing the troubleshooting steps below in the order presented.
Ensure your Pokémon GO app is up to date
Pokémon GO works best when using the newest version of the app. If you’re experiencing an issue with an older version of the app, please update your Pokémon GO app and see if this fixes the problem for you.
Open the App Store app.
Navigate to the Pokémon GO app and touch the Update button.
Check your internet connection
Ensure your device is connected to a reliable data connection (Wifi, 3G, or 4G). Try switching internet sources: if you have trouble connecting to Wifi, try to connect using 3G or 4G. If you have trouble connecting to 3G or 4G, try to connect using Wifi.
Use automatic system time
Make sure that your device's time and date is set to update automatically. You can update your system time using your device’s Settings app.
Open the Settings app on your device.
Under Settings > General > Date & Time, turn on the Set Automatically option.