Tickets bought via Billetto are only refundable if approved by the organiser of the event. The approval of refunds is entirely at the event organiser’s discretion, and you should get in contact with the event organiser to discuss what options are available to you.
A link to contacting the event organiser can be found just beneath the main picture on the event page itself. The organiser's email address can also be found in the confirmation email. Remember to include the Order ID of your ticket (which can be found on your ticket and in the confirmation mail) as this will help to process your refund quicker.
If you have purchased Booking Protection for your tickets, then you may be able to claim a refund via this link. The 7 digit Order ID (as can be found on your ticket and in your confirmation mail) is also your Booking Reference Number in this case.