You are running PHP 5.5. To allow you to upgrade to Nextcloud 11 and higher you need to run at least PHP 5.6. Once you upgraded your PHP version you will be able to receive update notifications for these newer versions

I get the message from the title. I’m on version now inside a freenas plugin jail.
Any advice on how can I upgrade to v12? And how can I update php?

You first need to upgrade the php version you are using to php 5.6 or higher. With php7 there is a noticeable performance improvement. Check the FreeNAS/FreeBSD community if you need help with that. After that you can upgrade to NC 11 and finally to NC 12. Don’t forget to make backups.

I’m not sure what update procedures work in your jail environment and how you installed it (via ports etc.). In doubt, manual upgrade are generally less problematic.