Hi Michal, so basically it was a theme bug so I had to contact them for them to solve it. This was the message they sent me before fixing it:
It’s not working because you didn’t have your variants block enabled. Since it doesn’t appear you have any variants anyway, I’ve enabled that block, it does not change the look of your shop at all, and now items can be added to the cart. We’re investigating that issue generally in the theme now.