1. Go to "Settings" on the phone;
2. Scroll down and tap "About phone";
3. Tap "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 "Developer options";
6. Turn on "USB debugging";
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.