Is your video iFrame showing YouTube error or Message Youtube will not play?
This is caused by the URL link format being a watch format, no worries it is an easy fix!
You must ensure the URL contains embed rather watch as the /embed endpoint allows outside requests, whereas the /watch endpoint does not.
<iframe width="420" height="315" src="https://www.youtube.com/embed/XFXGXFSF?rel=0&showinfo=0" ... >
Simply add the /embed into your watch URL and video should perform without any issue.
TIP: Add ?rel=0&showinfo=0 to end of your URL to remove Youtube video sugguestions !!