Debian 10 has now fully switched to Systemd which means that the traditional commands to halt or reboot a system like 'shutdown -h now' or 'reboot' will not work anymore. First of all, run the command as root user. Prepend the sudo command to become root user if needed. Here are their systemd equivalents which need to be used on Debian 10:
Shutdown Debian 10
The command to shut down (power off) a Debian 10 system is:
systemctl poweroff
Reboot Debian 10
If you want to reboot your server or Desktop, use this command:
systemctl reboot