Check out this tutorial from Vivek Gite on how to use jails on FreeNAS Corral. Jails was a feature in FreeNAS 9.10 and was replaced with Containers in Corral. Follow the link below for the full set of instructions.

recently installed or upgraded to FreeNAS Corral (version 10). However, I noticed that there is no support for FreeBSD jails as of 26/March/2017. How do I install and configure traditional FreeBSD jails on FreeNAS Corral (version 10)?

FreeNAS Corral now supports Docker containers for doing all of its “application hosting” and existing jails/plugins data will simply continue to live in the jails/dataset in the ZFS volume but will be inactive, since jails are no longer used in Corral. This tutorial shows you how to create and configure FreeBSD jail on FreeNAS Corral 10 from command line.

Full tutorial: https://www.cyberciti.biz/faq/how-to-install-and-configure-jails-on-freenas-corral-10/