Chromecast not supported in Alexa skill yet?
No available network chromecast was listed in available players. Additionally I launched the Plex android app on my phone (nexus 6P) and attached it to a chromecast on my network. I then had alexa list available players, and only the phone attached to the chromecast was available. Asking her to play to that player merely started the playback on the phone and not the attached chromecast. So it seems chromecast is not yet supported by this skill (confirm?) as well as using a player attached to a chromecase to as as a proxy for the skill also will not work (you would then need to re-attach to the chromecast for playback on it once it started playing on your device) at this time.