The problem is that CORE is dead, and its most recent release is based on a version of FreeBSD that is long since EOL at this point. Availability of binaries that will run under FreeBSD 13 is spotty at best.
Your options at this point are:
- If you want to stay on FreeBSD, look at moving to FreeCORE (there’s another thread here about it, in the General Discussion category). It’s a fork of TrueNAS CORE rebased to FreeBSD 15, so you’ll be able to create jails and install packages (like Plex) in them. It’s a new project, though, and appears to have just one maintaner, so how well it will be maintained is an open question. But since it’s FreeBSD-based, you’ll be able to keep your existing jails, upgrade them incrementally to a current FreeBSD version, and install new software in them. It also supports direct upgrade from CORE 13, though I don’t know about CORE 12.
- Move to TrueNAS SCALE and install Plex there, either using the TrueNAS Apps system or as a custom Docker app. iX isn’t likely to go anywhere any time soon, so SCALE is going to be maintained and updated for some time to come. I don’t have much faith in the Apps system, but I don’t expect Docker to go anywhere.