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.
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" ... >
Simply add the /embed into your watch URL and video should perform without any issue.