If a different version of FreeNAS is running from the version of a jail. For example, the FreeBSD version FreeNAS is based upon is 11.0, but a jail is based on 10.3, the following error can occur when attempting to install something from ports in a jail.
make: "/usr/ports/Mk/bsd.port.mk" line 1177: UNAME_r (11.0-STABLE) and OSVERSION (1003000) do not agree on major version number
There are two solutions to this problem, the non-permanent option is to set an environment variable in the current shell with
setenv UNAME_r 10.3-RELEASE.
The permanent solution is to set the environment variable by editing
/etc/login.conf, and adding the jail's version.
Reset the database.
Exit and enter shell and everything should work.