Hey,
In Unit, iFrame shortcode is asking two information of your iFrame code (the code you got from source).
iFrame Height: which is 600 in your case so enter 600 there.
iFrame url: this is the part you made wrong entry. you just need to enter url (the src part) here.
sample iFrame code:
here iFrame url will be : https://www.youtube.com/embed/tgbNymZ7vqY
so we just need to put this (https://www.youtube.com/embed/tgbNymZ7vqY) in iFrame url.
refer :
http://prntscr.com/lryuqj
if any further query please reply we will happy to help you.