It becomes irritating when cloning a server only to get the hostname error again.
root@server1:~$ sudo apt update sudo: unable to resolve host server1
This happens because the server is unable to determine the hostname of the server
Resolution includes looking at 2 files.
In /etc/hostname ensure the hostname of your server is updated, matches the current hostname of the server you are using.
sudo nano /etc/hostname
*edit file to hostname
Ctrl + x and y to save
and in /etc/hosts
sudo nano /etc/hosts
*edit file where server1 is your hostname
127.0.0.1 localhost 127.0.0.1 server1
again to exit, ctrl + x then y