Problem:
While trying to adb pair
to your device, you see an error message like
$ adb pair 10.9.1.104:34879 Enter pairing code: 232521 error: unknown host service
and the pairing fails
Solution:
You are not running the (correct version?) of the ADB server.
Run
adb kill-server
and then retry your adb pair
command.
If that doesn’t work, reboot your device.