Why did my app store app disappear?

The App Store app is not one of the apps that can be deleted but with iOS 14 it can be removed from the home screen. If you have iOS 14, go to the App Library (swipe left past your last home screen). From there you can touch and hold an app and slide it left back to the home screen.

Another reason for it not being visible is due to a restriction on purchasing. Go to Settings and tap Screen Time. tap Content & Privacy Restrictions. tap iTunes & App Store Purchases. choose a setting and set to Allow. See: https://support.apple.com/en-us/HT201304#prevent-purchases

If that is not the cause, try using Search to see if that will locate it: https://support.apple.com/en-us/HT201285