Hey everyone! I finally found a solution to the problem, the problem is with the last two packages of the plugin (They've been updated since being written on their GitHub page).
If you change the nextcloud.json packages to "security/py-fail2ban" and "security/py-certbot" (instead of py38-...), it should work just fine.
In case you don't want to mess with it, I forked it and uploaded to GitHub (and changed the README)
Official iXSystems iocage plugins for FreeNAS and TrueOS - GitHub - liamrahum/iocage-ix-plugins: Official iXSystems iocage plugins for FreeNAS and TrueOS
github.com
Enjoy!