First, make sure that your server conforms to the installation requirements: Software Requirements for Plesk Onyx.
Preparing the server
Assuming you are on a fresh install lets patch it first:
$sudo apt-get -y update && sudo apt-get -y upgrade && sudo apt-get -y dist-upgrade
sh <(curl https://autoinstall.plesk.com/one-click-installer || wget -O - https://autoinstall.plesk.com/one-click-installer)
If everything worked, you should see that provides you with a setup URL