systemctl does not see service apt install on Kubuntu 21.04 installed itself
Solution 1:
Neither of those packages install service files for you to use. To run these programs as daemons on startup, you will need to create the systemd unit files yourself.
Here are some instructions for x11vnc: https://askubuntu.com/a/676978/797559
Here are instructions for xvfb: https://superuser.com/a/912648/910217