One quick clarification first: your Mac is not a Nucleus - it is running Roon Server software, which is the “brain” of your Roon setup. A Nucleus is a dedicated hardware device made by Roon. You can read more about how Roon’s architecture works here: Roon Architecture and Roon Server.
Regarding your streamer not appearing: since you are on macOS, the most likely cause is that recent macOS versions apply stricter Local Network permission controls which can prevent Roon from discovering devices on your network. Please try the following:
Open macOS System Settings → Privacy & Security → Local Network
Make sure Roon and Roon Server are both enabled
Even if they are already enabled, toggle them off and back on
Fully quit Roon Server from the macOS menu bar
Reboot your Mac
After reboot, launch Roon again and check if your streamer appears
Please also let us know what streamer model you have - this will help us confirm whether there are any additional steps needed on that side.
Just checking in on this. Were you able to go into macOS Privacy & Security, confirm that both Roon and Roon Server have Local Network access enabled, toggle those permissions off and back on, then fully quit Roon Server, reboot the Mac, and see whether your streamer shows up afterward? We’d also still like to know the streamer model you’re using, since that can help us narrow down the next step. Please reply with any update when you can, thanks.