My Samsung QN900A 85" gives that "Available for specific video sites" message at the Chrome "cast" prompt. YouTube works fine but I haven't found any other source that will cast from there. There is no Chrome cast app on my TV and Smart Things doesn't connect to Windows.
Your question prompted me to look for another way. Setting up the TV as a wireless display turns out to work well: PC Settings>Display>Connect to a wireless display>Connect. I can now extend or duplicate the PC screen to the TV and the audio comes through the home theatre.
Thank you for your question. Maybe I can now hang up my HDMI cable...
Hope that helps.