Trying to connect Spotify to Chromecast from Android OS apps does not work. The Chromecast option can be seen but can not be selected. Underneath it says something like "Can not be played right now" (direct translation from Finnish) or such. I've reset and re-installed Chromecast, we have the latest Spotify updates and have also tried switching things on and off again. This has been an ongoing issue for a month or so and it is starting to get REALLY frustrating. The latest update at the end of June got things working for a few days and now we're back to NOT WORKING. Everything else works and connects just fine with Chromecast, so it is not a connection issue. It's the Spotify app.
If you're experiencing difficulties with casting Spotify from Android to Chromecast this issue could be related to one that we're currently investigating. Check out this Ongoing Issue and add your vote to it by clicking on the +VOTE button. Make sure you follow the steps provided under Status Update as well.
It'll also be useful to subscribe via the three-dots menu in the top right corner next to the title. That'll make sure you stay up to date with any relevant updates about this.
Hope this helps. Let us know if you have any more questions.