The page you link to <https://jitsi.org/downloads/> contains (mostly) instructions on how to install jitsi-meet on your server for a self-hosted WebRTC videoconferencing solution.

This does not include the older 'jitsi' named softphone / voip client, meaning there will be no jitsi command that can be executed.

Installation of jitsi-meet was straightforward for me, exactly as oulined on their page:

- first, add their repository to your apt sources.list

- then apt update && apt upgrade

- then apt install jitsi-meet

You should end up with a running web server hosting your own jitsi-meet instance.

I've installed it with a letsencrypt cert... you have to convert it to pkcs#12 with openssl and move it to /etc/jitsi/videobridge/ .