App Store

Check in Settings to see if you have any restrictions on. The App Store cannot be removed. If you cannot find it with a search on the iPhone, Use Search on your iPhone, iPad, or iPod touch - Apple Support then you can go to Settings>General>Reset>Reset Home Screen Layout.

The usual reason for the App Store app not being visible is due to a restriction on purchasing. See: Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support