1. Go to Settings;
2. Scroll down and find "About device";
3. Scroll down and find "Build number", tap it 7-10 times to make Developer options available;
4. Go back to the previous page and tap "Developer options";
5. Tap "On" and tap "OK" in the pop-up window;
6. Scroll down the page, find and turn on "USB debugging", tap "OK" in 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.