Cannot install plugin under Freenas 11.3

Hello I’m new so apologies if I’m just being stupid…

I’m on Freenas 11.3 U5 and am trying to install the nextcloud plugin. I get a message Error: 11.3-RELEASE was not found! so it does not even install.

Please can anyone tell me if there is something obvious I should be doing?

Thank you

I don’t know that system at all, but it seems to be based on FreeBSD 11, which is no longer supported. And FreeNAS as such does not seem to exist, it was merged with TrueNAS:

If you start installing something, I would strongly recommend to use a current operating system which still gets security fixes.