I have Libreelec on 3 devices. When I upgraded to 17 & got estuary, I did as the link shows, & "get more" to install other skins. I think confluence comes with the default repository. So it should be there. Are you connected to the internet. (sorry if it's a silly question)
I use Titan, I know you want confluence. But try it out
Titan skin version 3.7.x (Kodi 17 Krypton)