to the best of my knowledge iframe is not supported by Shopify due to security reasons
By default, Shopify enables the header to stop the shop being embedded into an iframe element. This is important to counter clickjacking attacks. If you have a good reason to be able to embed within an iframe consider contacting the support team to see if they can assist.
You do have access to things like the Storefront API (and others) if you need to show things on third party sites so perhaps that's another path.