Can't authorise android lenovo tablet in USB debugging mode
Resolution
Possible solutions if adb devices shows a device as unauthorized:

C:\Users\JohnDoo&gt;adb devices
List of devices attached
17310D821D  &gt; unauthorized


Disconnect USB between PC and device
Stop adb server by entering adb kill-server in command window
On device use Revoke USB debugging authorizations in Developer Options
On PC delete adbkey file in user directory, for example C:\Users\JohnDoo\.android
Reconnect the device to the PC
Open a command window and enter adb devices. Watch the device's screen for any Authorization message and allow the connection.

The device should now show as device:

C:\Users\JohnDoo&gt;adb devices
List of devices attached
17310D821D  &gt; device

If the above does not help, re-installing adb and or USB drivers may help.