Hi, well yes actually I pointed out the root cause but it wasn't coming from Freenas. When installing the plugin through GUI, Freenas will create the jail, make a DHCP request to obtain an IP address, then install and configure necessary packages ... However, I had set up a secure access on my Internet Provider router to manually authorize any new device on the lan. So my problem was coming from at the end that I didn't have Internet access when trying to install the plugin.
Hope it will help