Check that your link begins with ‘https://’ not just ‘http://’. The extra ‘s’ is a security feature, and without it most sites will not work properly in an iFrame due to browser security restrictions. Check the link (including the ‘s’) in a browser to make sure the website supports https.
iFrames are an advanced feature that rely heavily on the website you are inserting being formatted correctly. We generally recommend you only attempt to iFrame sites you control (and can make edits to in order to improve how they look in your Plattar app).