If you're looking for the CC button on YouTube TV, you won't find it in the same place as you would on the regular YouTube website. That's because YouTube TV doesn't have a dedicated website - it's only available through an app.
To find the CC button on YouTube TV, open up the app and start playing a video. Towards the bottom of the screen, you'll see a few icons - one of them will be a CC symbol. Tap on this and you'll be able to turn closed captions on or off.