@YouTube has to seriously figure its @Chromecast connection issue on the iOS app.
I have tried closing & opening the YouTube app 10 times and it still doesn't show the option available chromecast device.
Netflix connects without a hassle.
Thanks for the tag – usually, reinstalling the YT app fixes this. If not, we'd recommend sending a feedback report directly from YouTube, here's how: https://yt.be/help/feedback . Make sure to send from the account experiencing the issue!