If you are not able to backup or sync photos and videos even though Google Photos Auto Backup is ON, then try to switch Auto Backup Off and then ON start it up again. It this does not work, then try to reinstall the Google Photos app on your device. You can also try to Clear your app data. It is the same as uninstalling and reinstalling the app. Once you have clear your app data, sign in to your Google Photos app and turn ON "Back up & Sync feature.
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