Just another mention here about how poorly chromecast is supported by spotify. In addition to this issue which everybody seems to be experiencing, I get all sorts of other problems. Most notably, I am always needing to restart spotify on my android device (Nexus 5X android 6.0.1) when I want to connect to my chromecast audio. Here is a screen shot of what my "devices availible" tab looks like often. When I click on any of those "nearby devices" it asks me to reenter the device pin, but anything I enter is not accepted.

This seems to happen most reliably when I am listening to some music and then pause it for a while then come back to my phone to put it back on, but sometimes it happens when I get home from work and try to put something on. Almost always restarting Spotify fixes the issue, so I am fairly sure it is a Spotify issue.