I've been having some quirks in iOS 10 and App Store. I think it's because memory is low (20 gb remaining). Kept saying retry downloads.
Maybe you need to reset your network setting or reboot. Make sure cellular data isn't use a proxy or vpn.
I've noticed that when I try to purchase an app where the latest version requires iOS 11 it declines my purchase and refuses to take my money. Yet I sign in using another iOS device running iOS 12 and I'm able to purchase the app, then I return to my older device on iOS 10 is able to download the previous version. Stupid. Money is money.