Your Discourse site requires users to login in order to view its topics, and it’s on a subdomain of the discourse.group domain. I suspect you’re running into this issue: https://meta.discourse.org/t/embed-discourse-comments-on-another-website-via-javascript/31963?page=2#embedding-comments-from-a-private-site-9.
Unless something has changed, the only case I’m aware of when embedded comments from a private Discourse site can be viewed on a webpage is when the Discourse site is on a subdomain of the website’s domain, and the user who is viewing the webpage is currently logged into Discourse.
If this is correct, it means that it’s not possible to embed comments from sites on Discourse’s Basic plan.