How to reboot after power failure for Mac mini running Ubuntu

create a file /etc/systemd/system/reboot_on_power_failure.service

[Unit]
Description=Reboot after power failure

[Service]
Type=oneshot

# reboot register for Mac Mini with Intel ISA bridge
ExecStart=setpci -s 0:1f.0 0xa4.b=0

[Install]
WantedBy=sysinit.target

And run:

systemctl enable --now reboot_on_power_failure.service