As @Arkshine has noted, the Publishing Username needs to be set to an admin user’s username, not to their full name.
You haven’t posted your full API Key here, but you need to be very careful about posting anything that contains API Keys from your Discourse site. If it was my site, I’d probably generate a new API Key now and update the API Key setting on WordPress to use the new key.
@angus, would it be worth blocking out the value of the API Key setting in the plugin’s UI? Doing that might highlight the fact that the key needs to be protected. It would also make it easier to post screenshots of the plugin’s Connection status page.