This is a fairly common error: Search results for 'Failed to execute 'postMessage' on 'DOMWindow'' - Discourse Meta. It’s a long shot, but one possible cause of the error is not having filled out the “Username for topic creation” setting that’s found on the Discourse embedding settings page.
That shouldn’t cause a problem.
Something that can cause issues is if your Discourse site is private: Embed Discourse comments on another website via Javascript.