This happens because you don't have your hostname configured in the hosts file.
in my case I have the following hostname XserverA and I get the following error:
sudo: unable to resolve host XserverA
In order to solve it
We edit the hosts file with the following command:
sudo nano /etc/hosts
We will find the following information for example:
127.0.0.1 localhost
127.0.1.1 ubuntu# The following lines are desirable for IPv6 capable hosts
::1 localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
We will add our hostname as I put it:
127.0.0.1 localhost
127.0.1.1 ubuntu XserverA# The following lines are desirable for IPv6 capable hosts
::1 localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
You add yours, then close the editor with the keys Ctrl + X, then press Y then Enter.
