Why am I getting this reboot error on Ubuntu?

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'.