How to power off a Debian Linux box via console

You need to be root (or use sudo):

# This halts (shuts down) the computer now
sudo shutdown -h now

# This reboots the computer now
sudo shutdown -r now

Read the manual page for shutdown: man shutdown for more information.

In the link you gave, you just overlooked the sentence

simply run as root:

You need to be root (or use sudo):

To shutdow the pc use:

systemctl poweroff

To restart the pc use:

systemctl reboot