Set up Roon on my iMac and it doesn’t list the iMac as an audio device. It used to allow playback on the iMac but the iMac doesn’t show up as an audio source any longer.
Can you give us a screen shot of Roon > Settings > Audio?
One of the things that can cause this is related to Mac sleep settings. As odd as it seems, the Mac going to sleep and/or turning off the hard disks can cause audio devices to disappear too (System preferences, Energy Saver).