Have you read through this? WP Discourse plugin installation and setup
When SSO is enabled, the plugin doesn’t automatically log you in to the Discourse forum. What it does is outsource the Discourse login operation to the WordPress site. If it’s working correctly, when you are logged into the WordPress site, clicking on the Login link on Discourse should automatically log you into the forum. If you are not logged into the WordPress site, clicking on the Login link on Discourse should redirect you to the WordPress site. After logging in on WordPress you should be redirected back to Discourse as a logged in user.
The plugin also generates ‘start the discussion’ or ‘continue the discussion’ links at the bottom of each post. By default, clicking on one of these links should log you in to WordPress (if you’re not already logged in) and then redirect you to the WordPress site as a logged in user.