What FreeNAS version are you running is this the old system or already supporting iocage jails?
In either case you need to update to 11.2 that is our minimum supported version for FreeBSD, it seems to still work for a bit on 11.1 (or at least start) although not supported but that is no longer the case.
There more info why it no longer starts on 11.1 here: Server doesn't restart after 1.15.6.1079 update
But in any case the solution is to upgrade you’re system/jails.
