Allow USB debugging popup does not come in my new PC
I also got the same issue in my lenovo mobile. But when i deleted the .android folder in the home directory, my problem is solved.

For me 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 .android(Linux - /home/--username--/.android, Windows - C:/Users/--username--/.android) in the home directory of the current user which contains the private and public rsa keys used to communicate between the devices. 

When I check the .android folder, it is assigned with root permission(I am using Ubuntu 16.04 and I don't know how its assigned). So when I try to run adb start-server it is trying to create .android folder. But the current folder with root access is not allowing this to create the adb keys. That's why its failed.

So I deleted the folder using below command. Use a 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.


  adb start-server


Now It created the .android folder and the rsa keys with the current user permission. Once done this I got the Allow USB debugging dialog in my mobile. 

I know its too late. But I'm mentioning here because it'll help someone in the future.