Hey @Mathieu,
I struggled myself a bit to get this going and in my case there were 2 things I missed:
Make sure you enable the YouTube API as stated here (see #1):
https://github.com/bugsounet/MMM-Assistant2Display/wiki/YouTube
Copyrighted videos won’t play as stated here:
http://forum.bugsounet.fr/topic/53/youtube-can-t-play-videos-with-license-youtube-js-1-a2d-yt-error-150-not-allowed-by-owner/7
After enabling the API and asking the Assistant for a video without copyright it started to play.
Hope this helps ;)
Best,
PH1TCH