Substack embeddable subscribe button - code not working!

Hi - I am trying to embed my substack into my squarespace website (I have a business plan). When I add a code block and input the code (copied directly from substack) all I get is an unhappy grey box (see photo). I have followed these instructions: https://www.paulinewiles.com/articles/embed-substack-in-website.

Anyone know how to fix? 

resolved