Are you casting to a Chromecast device, or are you using the built-in cast on the TV?
If the latter, try updating the TV's Cast app on Google Play Store -> My apps -> update Cast Receiver App.
Yikes. Is there confirmation yet that Google Home works with THIS year's Sony Bravias?
It doesn't see mine either. But that's ok, because the TV sucks balls and is going away soon.
First commenter here says otherwise. Not sure which bravia.