From https://www.macworld.co.uk/how-to/iphone/what-do-if-your-iphone-cant-connect-app -store-3598048/
Okay, let's try some simple resets to see if this jogs the App Store into working. First, try logging out and then log back into the App Store on your iPhone - that should fix it:
- Go to Settings > iTunes & App Store
- Tap on Apple ID and Sign Out
- Log in again using your Apple ID and password
Perhaps it's a memory issue (older iPhones, especially those with less RAM and less storage are prone to this). Double-tap the Home button and swipe up on the App Store to close the app. Reopen it and see if that fixes the problem.
If it doesn't, close all open apps and turn your phone on and off again to clear the memory. You can find some other tricks to free up your iPhone RAM here: How to clear the cache on your iPhone or iPad.
You could also try force-refreshing the App Store app by tapping ten times on the App Store navigation bar (that's the one at the bottom of the screen that is populated with Featured, Categories, Top Charts, Search and Updates). You should see the app flicker, or go blank and display a 'loading' notification.
Your Date & Time settings are unexpectedly relevant to this issue, because the App Store uses them to check on your location (probably).
Go to Settings > General > Date & Time and choose Set Automatically. (This setting means that if you travel from, say, London to San Francisco, as soon as your iPhone recognises it is in a new territory it will automatically change the time accordingly.)