Check whether the App Store app is hidden in a folder or on another Home Screen page
Swipe down on the Home Screen and search for App Store
Open Settings
Tap Screen Time
Tap Content & Privacy Restrictions
Tap iTunes & App Store Purchases
Tap Installing Apps
Set it to Allow
If needed, go to Settings
Tap General
Tap Transfer or Reset iPhone
Tap Reset
Tap Reset Home Screen Layout
Restart the device
Update iOS to the latest version
If the App Store still does not appear, contact Apple Support