What OS do you have installed on the Pi? Is Roon Bridge installed? If the Roon Core doesn’t detect the Pi as RAAT output device, I am inclined to think that RAAT (Roon Bridge) is not present or not correctly configured. On the other hand, that Roon recognizes the Pi as Airplay device may indicate that you have RoPieee XL installed and have the Shairport part activated…
Ok, that explains the Airplay and lack of RAAT… I believe it is possible to install Roon Bridge over Volumio, though.
What makes you think so? The setup of RoPieee sometimes can be a little fickle, so I suggest to strictly follow the excellent Beginner’s Guide. One important point to be careful with is that for setup you must connect the Pi using an Ethernet cable. Once it is set up with WiFi enabled, the Ethernet connection is not required anymore.
By the way, if you don’t need Airplay or UPnP or Spotify Connect support on the Pi, you should install RoPieee instead of RoPieee XL. It is smaller and simpler, and if needed you can always later ‘upgrade’ it to the XL version from your Pi’s RoPieee web interface.
First thing is to make sure that RoPieee is really up and visible on your network. How do you have it connected? WiFi or Ethernet? You can try to go to http://ropieee.local if you don’t know the Pi’s IP. If you do know the IP, try to go to http://aaa.bbb.ccc.ddd. Does RoPieee’s web interface show up? Did you set up your Allo Boss DAC on this web interface? If not properly set up, Roon won’t be able to talk to it.