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