Maybe this read will help. If you can't connect to the App Store, iTunes Store, or other Apple services https://support.apple.com/en-us/HT201400. The document may be helpful generally in regard to your problem, but, perhaps especially so if the error you saw was "unknown error has occurred."
Use the steps above if you see one of these error messages:
· "Cannot connect to the iTunes Store. An unknown error has occurred."
· "We could not complete your iTunes Store request. The iTunes Store is temporarily unavailable. Please try again later."
· Could not connect to the iTunes Store. Make sure your network connection is active and try again."
· "We could not complete your iTunes Store request. The network connection could not be established. There was an error in the iTunes Store. Please try again later."