1. Go to "Settings" on the phone;
2. Scroll down and find "About this phone";
3. Scroll down and find "System Version";
4. Tap the Build number 7-10 times to make Developer options available. (You will see "You are now a developer!");
5. Go back to the previous page and find "Advanced Settings";
6. Scroll down and find "Developers Options";
7. Turn on "USB debugging";
8. Tap "OK".
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.