Dunno then, the only time I’ve seen something similar to this before is where the Spotify service within Sonos is logged in with an account linked to the ‘free’ Spotify service, but if you have the family plan setup and are logged in with the primary Spotify account, then it should be working for searching for particular songs.
I would maybe go onto do a (failed) search and then submit a Sonos system diagnostic report from within the Sonos App and either note/post it’s reference back here and then contact/chat with Sonos Support Staff via this LINK and see what the Staff can suggest to resolve the matter.