Don't have the App Store

Use App Library to search the App Store app, see -> Organize the Home Screen and App Library on your iPhone

Organize the Home Screen and App Library on your iPhone - Apple Support           

If you have find theApp Store app, tap & hold on the app, then drag to the Home screen & drop it.

You cannot delete the App Store app so it is on the phone or it is restricted in Screen Time. If you use parental controls on the phone, access to the App Store may have been restricted which would cause the app to disappear.

See this for more information.

Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

Check the App Library to see if it is there.