If you’ve enabled login verification on your account, be sure to store a backup code before removing your account from the app.
Also, if you are downloading or updating apps over Wi-Fi or 3G make sure the problem is not with your data connection before proceeding.
Now look for the DNS and input 126.96.36.199 (is a Google Public DNS– which will ensure safety and speed) and make sure your i OS 10.3.1 device has enough space to download.
If your i Phone 6 won’t download Apps still, try restarting the device so the internet would reconnect.
Make sure you are signed in with the same account that made the purchase.
When you locate the app in the i Tunes Store, you should see “install” or “Downloaded” instead of a price.
Even with the latest update on ios 10.3.1, this error would appear sometimes.