Try enabling the WP Discourse Send Email Notification on Publish Failure setting. It’s found on the Publishing tab of the plugin’s options page. When it’s enabled, you should be sent an email with some more details about why the post failed to publish.
Other things to check:
- make sure the plugin’s API key setting is using the Discourse All Users API key
- the Publishing Username should be set to the username of a Discourse admin user (try using ‘system’ if you haven’t renamed the system user)
- make sure your Discourse username is set on WordPress to be exactly the same as your username on Discourse. The Discourse Username is set on WordPress on your profile page, in the Contact Info section.