Ok, I must be too stupid to do this. I have my Sonos system added to ST. I add the Mood Music smartapp. The only option I have is to select a track. I have no option or ability to select any Sonos favorites, such as an Apple Music playlist I added to Sonos Favorites.
The theory used to be that it had to be playing but then it just stopped working for me. Now I just use Yonomi, which is also hit or miss but more hit.
Try (1) playing the Playlist or station on the Sonos speaker, (2) in the ST app, refresh the Sonos device so that it reflects what you’re playing, and then (3) try to use that Playlist or station.