when you install Debian with just the Standard minium System utilities a power off by pressing the power button is not possible.
To enable the power button installed the acpi daemon. The acpi daemon handles the acpi events(which a powerbutton raises).
root@debdev ~# apt-get -y install acpid
Then you have to tell systemd to shutdown the PC. Edit /etc/systemd/logind.conf and uncomment
Possible other actions are “ignore”, “reboot”, “halt”, “kexec”, “suspend”, “hibernate”, “hybrid-sleep”, and “lock”. See systemd logind help page for further details.