Hi
Go to iTunes main page bottom of page apple I'd click that view I'd Go to manage payments check your card details are correct and up-to-date.
Is your Internet working turn off your router for 20 /30 seconds when it's back on go to settings on iPhone down to reset Reset network settings After reset enter your router wireless key.
Try & download another apple that one may need updating to work on ios 11
Cheers
Brian 🇬-1F1E7;