I am trying to create a sample addon which have an iframe. I am getting the following error:
The value returned from Apps Script has a type that cannot be used by the add-ons platform. Also make sure to call build on any builder before returning it.
You can't use iframes with the Gmail add-on. You can only build cards using google apps scripts. Here are some more restrictions https://developers.google.com/gmail/add-ons/guides/restrictions