Good evening Matteo and welcome to the Events Calendar forum!
Thank you for your interest in our products. It’s truly appreciated.
However, please note that we do not offer support for our free plugin in this premium add-ons forum.
But, the good news is that you are still welcome to consult this forum to your liking and browse it for solutions, snippets and tutorials.
If you don’t mind, in the future could you please post your support questions here: https://wordpress.org/support/plugin/the-events-calendar or perhaps consider the purchase of one of our premium plugins to get access to premium support ?
With that in mind, it appears that no ticket RSVP ticket has been created for that specific event. You need to save the ticket before saving the event itself.
Additionally, if you are looking to sell tickets through WooCommerce, you will need to grab a copy of Event Tickets Plus: https://theeventscalendar.com/product/wordpress-event-tickets-plus/
Best regards,
Geoff B.