What you could TRY to do, is the following. Start the Spotify app on your Phone and have it play via your Chromecast. So that the Spotify app is actually running on your Chromecast. Then try to use Homey to connect to that running app. If both Spotify and Athom have done their work properly AND there are no limitations between your Homey and your Chromecast (firewalls or whatever) then Homey should be able to connect to it.
Yeah, I tried this - no, doesn’t work 
I’m using the latest version of Homey and Spotify v4.0.7 is installed. I have tried getting my Harman Kardon Citation One to work in flows but no go.
My units are showing (2 HK Citation One, Chromecast [gen1] and GHM) but when I push either Citation to play I just get the error message shown below. Very frustrating. Especially since Homey is marketed as:
All your music in one place
Homey plays your favorite music to any speaker. Automatically start playback with Flow.
Stream from Spotify, SoundCloud and Google Play Music to Sonos, Denon Heos, Chromecast, and many more.
