To install the latest PlayOnLinux version Ubuntu/Mint19, follow these commands:

wget -q "http://deb.playonlinux.com/public.gpg" -O- | sudo apt-key add -
sudo sh -c 'echo "deb http://deb.playonlinux.com/ bionic main" >> /etc/apt/sources.list.d/playonlinux.list'
sudo apt-get update
sudo apt-get install libgl1-mesa-glx:i386
sudo apt-get install playonlinux