What if..

the store registration fails

This issue is usually related to the REST API created by ¬†WordPress. The problem could reside in the permalinks, which is not allowing the API to work. Try to go to on the admin panel, to Settings, then Permalinks. There, on default settings radio group, select the “Post name” one. Save changes to conclude. If the solution doesn’t work, the issue could be also related to the file .htaccess.

You can't log to your store

When the connection is established, the app request to log-in into your store via the WordPress credentials. Make sure you are using the email of the account you registered to WordPress. You can find the email in the section user, profile, contact information, then email.