Verified again on bionic - 1.6.11, as the previous verification missed the version number.
$ systemctl reboot
Operation inhibited by "APT" (PID 2015 "apt", user root), reason is "APT is installing or removing packages".
Please retry operation after closing inhibitors and logging out other users.
Alternatively, ignore inhibitors and users with 'systemctl reboot -i'.
Verified again on bionic - 1.6.11, as the previous verification missed the version number.
$ systemctl reboot
Operation inhibited by "APT" (PID 2015 "apt", user root), reason is "APT is installing or removing packages".
Please retry operation after closing inhibitors and logging out other users.
Alternatively, ignore inhibitors and users with 'systemctl reboot -i'.