I’ve done what I was told by TrueNAS-13.2 Core, to install TrueNAS-13.3 Core in order to install apps. This was also subject in several posts here. Despite after all the upgrade, TrueNAS tells me: Error: 12.4-RELEASE was not found!# and refuses installing apps. Of course not 12.4! It was 13.2 now is 13.3 but apps do not install. What next? Thanks!
Simply stop try use plugins (deprecated from months) 
Create your own jails (there are a lot of community script that works really similar to a plugin) after an iocage fetch… or evaluate to sidegrade on Scale