How to Fix: USB Debugging Option Greyed Out

Solution 1: Unplug the USB cable before opening USB Debugging

It is recommended to enable USB debugging before plugging your device in to the computer. Unplug device then try to enable USB debugging. This should work. If so, you can then plug it back into the computer and it should work

Solution 2: Selecting the default mode as Internet connection

When you connect your phone to your PC using the USB cable, try selecting the default mode as Internet connection. Go to Settings -> Connectivity -> Default Mode -> PC Software. Then, enable the USB Debugging mode under Applications -> Development -> USB Debugging.

Solution 3: Use USB debugging mode on a device running KNOX (For Samsung )

By default, My KNOX and KNOX Premium (through CellWe EMM) disables USB debugging on a device when it creates a secure container.

IT admins KNOX Premium (through CellWe EMM) can later enable USB debugging mode through the Admin Portal: Simply select Policies > Mobile > Samsung KNOX Device Settings > Restrictions Settings > Permit USB debugging = Yes. On the device, you need to go to Android Settings, enable Developer options, then enable USB debugging. The exact steps vary depending on your Android version, as described in Using Hardware Devices.

If you are using My KNOX, you cannot enable USB debugging mode while the container is installed. Unfortunately, you have to root your device and uninstall 'My KNOX' (enter Knox, chose Knox settings app -> More Information -> unistall Knox.), then you will be able to turn on USB debugging mode.

If you are using KNOX Workspace with another, non-Samsung MDM solution, please consult the MDM documentation or Support for details about how USB debugging is managed.

Source: http://www.syncios.com/android/fix-usb-debugging-grey-out.html

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