I’ve just installed the wp-discourse plugin in my website https://covstat.it.
I’ve just mounted a hosted Discourse forum at https://forum.covstat.it, it seems to be ok.
I put the discourse forum URL + API key + admin username in the Connection tab of the plugin and… it simply work.
If you are not seeing a “You are connected to Discourse” success message at the top of the WP Discourse Connection settings tab, try generating a new All Users API Key on your Discourse site. Enter the new API Key into the plugin’s API Key field and enter system as the Publishing Username. After saving the changes, you should see this notification: