Error message “sudo: unable to resolve host (none)”

Solution

Two things to check (assuming your machine is called my-machine, you can change this as appropriate):

1. That the /etc/hostname file contains just the name of the machine.
2. That /etc/hosts has an entry for localhost. It should have something like: 127.0.0.1 localhost.localdomain localhost 127.0.1.1 my-machine

If either of these files aren’t correct (since you can’t sudo), you may have to reboot the machine into recovery mode and make the modifications, then reboot to your usual environment.

Refer: https://askubuntu.com/questions/59458/error-message-sudo-unable-to-resolve-host-none