Ok, I see. Thank you. The problem in my case is with the billing adress and the delivery adress (this is the same form actually). This setting do not allow the customer to order to the specific place indeed. In fact, I do want to allow them to purchase items from my website but the delivery can't be in the restricted states. The fact that the form is the same for billing and delivery adress is a bit problematic.
Anyway, thanks again !