>>43799
Nah, I figured out what was the problem, and can replicate it.
First time when I logged in and wanted to turn the machine off, as a user I tried to execute the shutdown now command. As the result of the mistake I got the command not found error. Then using su switched to root, but was the same result. At this point I looked up the systemctl poweroff, and moved on thinking I have to use that from now on.
Turns out:
1. when I try the shutdown as user first, I'm gonna get the error as that "pseudo" root the su switches into;
2. if I try the shutdown command as "pseudo" root, without making the mistake of trying it as a simple user first, then it works fine;
3. moreover when I try the shutdown as user first, and then switch to "real" root with su - or su -l then that command will work regardless the mistake;
4. systemctl poweroff works as a simple user too.

I'm not sure how that pseudo root is called.