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 Google Play Store app.
On the top left, touch the menu button.
Touch the My apps & games menu option.
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.
Visit the Settings app.
Under Date & Time, turn on the Automatic date & time and Automatic time zone settings.
Click the Remove button to disconnect Pokémon GO from your Google account.
To reconnect your Google account, log into Pokémon GO as you normally would. Open Pokémon GO on your phone and log in using your Google account.
Remove your Google account from your device (Google accounts only)
If all other steps do not resolve your issue, try removing and adding your Google account to your device.
When you remove the account, your email, contacts, settings, and other information associated with the account is removed from your local storage until you add the account back to your device. Be sure that you complete these steps with adequate time to allow for your account data to download.
Remove the email account from your device:
Open the Settings app on your device.
Touch the Accounts menu option.
Touch Google from the list of accounts. If you have multiple Google accounts, select the Google account you used to log into Pokémon GO.
Touch the three dots menu at the top of the screen, and then touch Remove account.