You must bring FreeBSD in the jail to 13.2 which is the lowest currently supported release. For anything older packages are no longer provided - that’s why you cannot upgrade anything else in that jail.
Now to be able to run a FreeBSD 13.2 jail your host needs to run TrueNAS CORE 13.0 (-U6.1 recommended). Does it?
Probably best would be - assuming you are running a current TN CORE - to create a new FreeBSD 13.2 jail, then follow this procedure to install mineos:
You might need to adapt a bit - the instructions are sometimes a bit outdated so some package might have to be replaced with a newer version. But generally I was always able to more or less follow through.
There’s also this resource by @victor which might be much easier - I never tried it:
Once you have MineOS up and running simply copy over the server directory. That’s not difficult but if necessary I can assist.
Most important: don’t bother to try the plugin, create a standard VNET jail without NAT bridged to your local network, then try to get MineOS up and running.
HTH,
Patrick