One possibility: If iTunes Store is restricted in Settings > General > Restrictions > iTunes Store then the app icon will not be displayed.
Another possibility: search for the iTunes Store app by using search.
and if the iTunes Store app was deleted:
See the following from Remove built-in Apple apps from the Home screen on your iOS 10 device or Apple Watch - Apple Support
- On your iOS device, go to the App Store.
- Search for the app. Make sure that you use the exact name of the app. Find the correct name of built-in apps.
- Tap to restore the app.
- Wait for the app to restore, then open it from your Home screen.