Hello,
It should be possible. I just gave it a go, and it worked for me in Chrome, Firefox, and IE11. What browser are you using and is the YouTube video public?
Here's what worked for me:
1. Open YouTube
2. Copy the URL for the video from the Embed code snippet they provide. (You only want the URL, any other tags will cause a syntax error.)
3. I pasted that URL in the our iFrame URL input:
4. I changed the height to 600, hit apply, and then saved the site for good measure.