If you look in the browser’s development tools you’ll see this error:
Refused to display ‘https://www.quora.com/‘ in a frame because it set ‘X-Frame-Options’ to ‘SAMEORIGIN’.
This means that Quora does not allow itself to be used in iframes. If you’re just testing, then try another URL. If you actually want to embed Quora, then I’m afraid that doesn’t appear to be possible.