These two commands are SysV-era commands; they're only “emulated” on modern systems:

Shutdown is just a symbolic link to systemctl, which “knows” that when it's invoked as shutdown should trigger the same actions as systemctl poweroff. Same for reboot/systemctl reboot.

So, either you simply replace shutdown with systemctl poweroff, and reboot with systemctl reboot, or you just add aliases (alias shutdown=systemctl poweroff).

# systemctl poweroff
# systemctl reboot