Happened to me at first too. Refreshing the page worked. Not sure if that's an intentional pause or not.
I think it's loading. My wifi LED was flashing furiously while that message appeared.
I'm having the same issue. I can't start it.
Edit: Keep refreshing the page to get it to load.
Wait a while and then the wear-headphones notification may eventually drop down off the screen. If so, then press the arrow keys and the thing may start playing.