Installed PI4J on Ubuntu Server 20

[raydacosta]

How should I proceed to run PI4J on Ubuntu Server 20 installed on Rraspberry 3 B+

[taartspi]

I only use the raspberry OS. But looking at the pi4j web site you are directed to https://github.com/thibmaek/awesome-raspberry-pi/blob/master/README.md. I do not see an ubuntu server for the pi4. Possibly that site where you were directed has a forum to ask about your OS usage