I am unable to download any app from App Store.

There are just a few things left to try and if these don't resolve the issue, then it would appear your only option would be to contact Apple directly:

• Is there storage space left when viewing at Settings > General > iPhone Storage?
• Do you have any VPN service on your phone?
• Have you tried another Wifi/Cellular network?
• Do you have a valid payment method? Normally you will get a message if it is a problem with your payment method, but I would still make sure there has been one added to your wallet.
• Try "Reset All Settings" option. This does not affect your data, but will remove all Apple Pay cards along with changing all the settings back to their default state. The previous settings can be restored with a backup. Reset iPhone settings to their defaults - Apple Support

Apple can be contacted directly here: Contact Apple for support and service - Apple Support

• What happens when you try?
• Is the Get button greyed out?
• Does it appear to start downloading, but never finishes?
• Have you tried restarting your iPhone?
• Any trouble with your Apple ID when signing in on website? https://appleid.apple.com