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Is this a fresh "out of the box" iPhone 6s or did you purchase it from someone? The App Store can be disabled through restrictions -- I've done this for the kids devices to keep them from adding apps. If it's a second hand phone, that may be a possibility.
It's also possible that the App Store is in a folder somewhere? If you pull down on the screen (rather than tap an icon, just put your finger in the middle of the screen somewhere and drag down) you'll get a Search option. You can type "App Store" there and see if it shows up for you.
If that doesn't work you may need to reset your device. This will erase everything and start fresh, which I can pretty much guarantee will bring it back. Go to Settings > General > Reset > Erase All Content and Settings