Why can't I find the device button in iTunes?

Recently, I just updated my iTunes to the software 12.5.1. Everything is running smoothly, except that when I connect mine devices (iPhone 4 and iPad Air 2, running on iOS 7 and 10 respectively) to the iTunes, the device button doesn't appear in the top left hand corner of the screen as usual. Why is that so?

Without that device button, there's no way I can sync or even back up my devices. Please help