New C8-Pro (2.4.3.137). No devices installed.
Devices / compatible devices. "Enter partial device name" produces no results. Brand drop-down finds lists Sonos, but selecting same produces no results. Scanning through the list finds Sonos. "Click the Install App button below" appears - no Install App button visible.
Interesting -- I'm seeing what you don't:
What browser and device are you using? If a mobile device (phone in particular), you might need to scroll around to see it.
But you don't need to do this at all. Just install the app yourself by going to Apps instead of Devices. Some devices, Sonos included, use an integration app to add the devices rather than being added through the typical "pairing" process that is necessary for Z-Wave, Zigbee, Matter, and similar devices like you'll find on the Devices page. So, it's simpler to just start there yourself.
Those steps are detailed here:
That should work for you!
Chrome (latest - 142.x), desktop, no button.
