The Webmin site seems to have slightly outdated installation instructions.
On Ubuntu 20.04/22.04:
wget -q -O - http://www.webmin.com/jcameron-key.asc | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/webmin.gpg > /dev/null
On Ubuntu 14.04/16.04/18.04:
wget -q -O - http://www.webmin.com/jcameron-key.asc | sudo apt-key add -
Then add the repository:
sudo sh -c 'echo "deb http://download.webmin.com/download/repository sarge contrib" >> /etc/apt/sources.list.d/webmin.list'
Update software:
sudo apt update
Install:
sudo apt install webmin