If an user which is loged in to his FreeNAS server over ssh and which is then trying to login as a root typing "su -" command gets

palo@PS ~/Documents/OneDrive $ ssh palo@192.168.1.5
palo@192.168.10.7's password:
Last login: Sat Nov 12 09:12:00 2016 from 10.0.0.17
FreeBSD 10.3-STABLE (FreeNAS.amd64) #0 r295946+07c41cd(9.10-STABLE): Wed Nov  9 00:19:25 UTC 2016

    FreeNAS (c) 2009-2016, The FreeNAS Development Team
    All rights reserved.
    FreeNAS is released under the modified BSD license.

    For more information, documentation, help or support, go here:
     http://freenas.org

[palo@freenas] /% su -
su: Sorry

This mean he is unable to get into the root account. Be able to login as the root that user have to be added into the "wheel" group. Than it works

[palo@freenas] /% su -
Password:
[root@freenas] ~#