Ctrl-Alt-Del to shutdown Ubuntu server.
ln -s /lib/systemd/system/poweroff.target /etc/systemd/system/ctrl-alt-del.target then reset systemd daemon: systemctl daemon-reload
A scratchpad of useful tips
ln -s /lib/systemd/system/poweroff.target /etc/systemd/system/ctrl-alt-del.target then reset systemd daemon: systemctl daemon-reload