1. Go to "Settings" on the phone;
2. Scroll down and find "More settings";
3. Go to "About phone";
4. Find "Software version, tap it 7-10 times to make Developer options available;
5. Go back to the previous page and go to "Developer option";
6. Turn on USB debugging and tap "OK" on the pop-up window.
Now you are ready to run non-root setup to remote control your device. (How to run Non-Root setup to remote control Android Phone with AirDroid?)
If you have done all the above steps, but still can't turn on the USB debugging of your device, please kindly submit a ticket.