The App Store app, has disappeared from my phone

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. 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