You can’t do what you’re trying to do. The Chromecast binding can send video/audio streams to a Chromecast, but you’re just telling it to play a URL for a webpage. So it’s trying to do that and can’t find any media.
There’s a trick to get it going, but I don’t know whether or not it works. I haven’t tried it, I just searched the forum for “Chromecast youtube”.
You could also try getting the actual video stream that youtube is using to deliver the video to your web browser.