This is because you already were in the root session.
Seems like 13.1-RELEASE is no longer available, and it’s a known issue:
13.2-RELEASE could be used but it isn’t available either. 13.3 is available but cannot be used with 13.1 host due to compatibility issues with daemon utility: it silently fails to execute the child.
So at this point you can wait until TrueNAS Core 13.3 is released, or install BETA2 – I’m running it for a few weeks now, it seems to work fine for my use cases (jails, replication, samba). You can always revert back to the previous boot environment if breaks either of your use cases.
You would need to edit the “release” to 13.3-RELEASE in the plugin json file before feeding it to iocage. I’ll update the repo when Core 13.3 is officially released.