Swipe down from the top of the screen to open the notification tray. From there, you should see an auto-rotate icon. Is it highlighted in a blue shade, or is it just in black lettering? If memory serves me correctly, the home screen does not rotate on the GS7. Make sure the icon is toggled and set to auto roate and then test it in youtube. If the icon in the notification tray says "portrait" or "locked" this could be causing the issue. The GS7 also allows search in the settings application if you do not see it in the notification tray. Simply start typing "auto-rotate" in the search bar and that setting should pop up.