TrueNAS CORE: plugins won't install - Syncthing, Nextcloud

Plugins are official dead and deprecated, as will CORE be in a few years unless IX decided to update, or someone decides to fork. You can either wait for Electric Eel because of this The Future of Electric Eel and Apps

Or install nextcloud using Scripted installation of Nextcloud 29 in iocage jail

And create a manual jail and install syncthing there.