Portrait Mode

There is no portarait mode on iPhone 7. You can use Portrait mode on iPhone XR, iPhone XS, iPhone XS Max, iPhone X, iPhone 8 Plus, and iPhone 7 Plus.

Turn off lock screen. Rotate the phone so it goes into portrait mode, then lock the screen on that if you wish to keep it that way

On the regular 7 it's not available. Only on the 7 Plus. (and 8 Plus and all iPhone X models). The Portrait Mode requires a dual camera (except on the Xr)