After updating to the latest operating system, there have been a few issues when connecting to the App Store.
How to fix App Store Is Unavailable or Down in iOS 11 on iPhone/Ipad:
- One simple solution we can do is to Sign out and Sign in again.
- Enable your Cellular data for App Store. Go to Settings > iTunes Store & App Store > Enable Cellular Data
- You may Set Date & Time automatically. Settings > General > Date Time > Select Set Automatically