Spotify on Android doesn't always find the chromecast device. It usually helps killing the application (force stop) and restarting, but not always. Sometimes restarting the phone is the only way to go. Google home app always finds the chromecast device.
Using Android 7.1.1 on Oneplus 3 and Spotify 18.104.22.1683. This has been around for quite some time during several versions and is quite annoying.
We're closing this thread as we haven't heard from you in a while. If you're still having the same issue, just give us a shout.