I understand that you're unable to download an app from the App Store, and I'm happy to help. Please try the following suggestions.
1. Test downloading the app on all networks. (Cellular, Wi-Fi, different Wi-Fi network)
2. Test downloading other apps, does it occur with one app or all apps?
3. Test if there are other devices in the household also have problems downloading that app or any other apps.Thanks for using Apple Support Communities.