I also got the same issue in my lenovo mobile. Initially I thought the issue is with my mobile.
But actually the issue is the
adb don't have enough permission to read the
rsa key to communicate with the device. When we run
adb server it'll create a folder named
/home/--username--/.android, Windows -
C:/Users/--username--/.android) in the home directory of the current user which contains the
adb rsa key files used to communicate between the devices. When I check the folder is assigned with
root permission(I am using Ubuntu 16.04 and I don't know how its assigned).
So I deleted the folder using below command. Use the particular method to delete the folder based on your operating system. Below I mentioned for linux.
sudo rm -rf .android
Then restarted the
adb server using below command.
Now It created the
.android folder with the current user permission. Once done this I got the
Allow USB debugging dialog.
Hope it help someone.