@PhilBr wrote:
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):
https://support.roku.com/article/360009649613
