Unfortunately iFrame content is no longer possible on many websites. In recent years there is a new setting which websites can add to the configuration of their web server which denies other websites from loading their content in an iFrame. This was added mainly to protect from malicious attacks and to direct traffic to their website only. This new server configuration is called “X-Frame-Options”.
Here’s an article which outlines technical details about what X-Frame-Options actually is, just in case you’d like to learn more about it: https://www.keycdn.com/blog/x-frame-options/