P.S. The tutorial is a bit outdated it seems, because it uses PHP 7.0 which is out of support. Replace with PHP 7.2 probably.
Or use the search engine of your least distrust to find a more up-to-date tutorial.
My point is: how to install $ANYTHING in a FreeNAS jail is generally not a FreeNAS issue at all. Once the jail is up and running, FreeNAS' job is done. You should refer to the particular application's installation manual which frequently enough even covers FreeBSD nowadays or is generic enough to be easily applicable. Like e.g. Nextcloud with their next to perfect Nginx config snippets that are mostly copy, paste, enjoy ...
How to install e.g. Nginx in the jail is not a FreeNAS topic. We as a community try to help as much as we can but on many occasions there are just better platforms - like probably Wordpress forums in your case.