Missing iTunes store app on iPhone
Go to Settings > General > Restrictions and confirm that restrictions are off. If restrictions are on, you'll need the restrictions passcode to turn it off.

Also, confirm that the top of settings doesn't have text mentioning "This phone is supervised and managed by..." at the top. If it does, talk to your company's IT specialist for more detail.

Lastly, on the home screen, pull down in the middle of the screen to reveal the search menu. Type "app store" and confirm that it doesn't show up there. If it does, it's likely in a folder or you simply missed it.