Install Webmin

Enable root access using sudo:

sudo -i

Edit the /etc/apt/sources.list file on your system:

nano /etc/apt/sources.list

Add the line:

deb https://download.webmin.com/download/repository sarge contrib

You should also fetch and install my GPG key with which the repository is signed, with the commands:

cd /root
wget https://download.webmin.com/jcameron-key.asc
apt-key add jcameron-key.asc

You will now need to install the apt HTTPS transport with the command:

apt install apt-transport-https

You will now be able to install webmin with the commands:

apt update
apt install webmin

All dependencies should be resolved automatically.