Thanks for posting here in the Google Nest Community.
To confirm, you are trying to add your TV to the speaker group. If that is correct, this feature is not supported for your TV at the moment. You need to contact Sony for this so they can create an update on their TV. Chromecast built-in is open-source software, so they are free to modify it as they see fit, and that could create any bugs or issues that only Sony would be aware of.
Let us know if you have other questions and concerns otherwise, we will be locking this thread after 24 hours.