Hi there!
There is no way to work around this specific error unfortunately. This is a security restriction imposed by web browsers that disallows iframes on a different origin to access the contents of the main window. Without this restriction, malicious code could be loaded in an iframe which would then have access to the main window, which is a huge security risk.
What you can do however, is use the postmessage API to send messages between windows. The only drawback to this is that the data sent must be serializable, so you cannot pass DOM elements. You may need to rework your code a tiny bit to make this work, but it is an option.
You could do some kind of flow where when your #sign-now button is clicked, you can post an “onClick” message to the iframe window, and handle the event in there.
Hope this helps,
Logan