If you receive the following error during Virtualmin installation or php7.2 installation on Ubuntu 18.04, it is due to the lack of necessary repository paths in source.list.
nano /etc/apt/source.list
In this section, the necessary repositories are loaded.
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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