Hey, so because Google decided to make it so I can only cast to Chromecast devices, I decided to use the Google home app to cast. I connected my Roku and all was good. When I pressed the power button on the Home app (which was weird in itself since my TV was on) my Roku said "thinking" then did nothing. Can someone help me with this?
Here I am in September 2020 with the same problem....
Not exactly sure what your issue is, however, be aware that Roku does not (directly or officially) support Chromecast (Googlecast) protocol -Roku supports DIAL and Miracast (neither of which is currently compatible with Googlecast Protocol, as per Google's intent).
The device/app needs to support DIAL (to cast) or Miracast (to screen share/mirror) when connecting to Rokus - so unless and until Google Home supports either/both of those, it wont be able to cast/mirror to any Roku device/app.
If you are trying to cast, you need to do this from within a DIAL-supported app (e.g. NetFlix, YouTube) via the cast icon (all your Rokus should show up as cast-able, assuming they are on and on the same network).
If you are trying to mirror/screen share, you need to do this from the device's Miracast-supporting mirroring/screen (e.g. SmartView for Samsung phones/tablets) sharing functionality (all your Rokus should show up as mirror-able, assuming they are on and on the same network).
Otherwise, Google Home will allow "voice control" of Roku devices via Google Assistant (with limitations and exclusions):