Try the steps below to fix problems with the App Store.
Switch off the App Store app and switch it on again.
Switch off your iPhone and switch it on again.
Check whether iOS is up to date and has the newest version.
We would also advise you to clear the data and cache if you still can't download the app:
Go to "Settings" > tap "iTunes & App Store" > tap the Apple ID > tap "Sign Out" in the pop-up > tap the Apple ID again and sign in with your Apple ID. Now, run the App Store to download apps to check the problem is solved or not.