If you receive the following error when installing the Virtualmin control panel or installing php7.2 on Ubuntu 18.04, the reason is that there are no necessary repository paths in source.List.
nano /etc/apt/source.list
Necessary warehouses are added. Use the commands below.
deb http://archive.ubuntu.com/ubuntu bionic main multiverse restricted universe
deb http://archive.ubuntu.com/ubuntu bionic-security main multiverse restricted universe
deb http://archive.ubuntu.com/ubuntu bionic-updates main multiverse restricted universe
sudo apt update; sudo apt upgrade -y
sudo apt install php7.2-mbstring
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