I have already made a forum contribution, but I'm happy to do it again here.
No plug-in can be installed under Truenas Core 12.0 U2.1 although everything is ok with the network. As soon as a jail is created, you can no longer perform name resolution within the jail and unfortunately the installation then fails.
I tried the same scenario under TrueNAS Scale, no problem here. Then I tried a TrueNAS Core 12.0 U2.1 Fresh install again, here again the problem with the DNS resolution. So I think the problem only affects the FreeBSD solution so far. I hope you can fix the bug quickly, thanks.