I am getting this, on sudo reboot on Ubuntu:
Operation inhibited by "nexsonic" (PID 2889 "gnome-session-s", user nexsonic), reason is "user session inhibited".
User nexsonic is logged in on tty2.
Please retry operation after closing inhibitors and logging out other users.
'systemd-inhibit' can be used to list active inhibitors.
Alternatively, ignore inhibitors and users with 'systemctl reboot -i'.
It could be that some daemon or background process is using tty2, as shown in that message, or that you inadvertently opened another tty session. For example, pressing CtrlAltF1 opens tty1, CtrlAltF2 opens tty2 and so forth. CtrlAltF8 returns you to the current desktop session.
Each session is independent of the other. For those who remember time-sharing computers, it's as if different users were logged on through different terminals.
Confirm if a task is already running on tty2 by pressing CtrlAltF2 to open that session. If so, then end that session before rebooting. Again, to get back to the desktop, press CtrlAltF8.