Hello Giovanni,
Sorry for the inconvenience you're experiencing. I understand it can be frustrating when things don't work as expected.
Please be aware that the behavior you're seeing is standard Joomla functionality. The system only permits previews of published articles. Unpublished, suspended, or trashed articles cannot be previewed from the backend. This is a Joomla core limitation, not related to any JoomShaper product.
As you've discovered, publishing the article will enable the preview feature.
Best regards