Imagine you are going to a concert. To get into the concert hall, you need a ticket beforehand, maybe you buy one from Ticketmaster. On the day of the concert you go and present your ticket to the ticket checkers and they make sure that your ticket isn't fake, hasn't been used and has been paid for. The concert venue wants to ensure that only people who bought valid tickets can enter the concert venue, to prevent cheating and fraud.
Supreme's checkout system is built roughly the same way. In order to check out on Supreme's website, you need a valid ticket(represented on your computer as a set of special cookies). If these cookies are invalid, were used before or missing when you go to checkout, Supreme will decline your order.
Because SwiftSole is in the business of checking out on Supreme's website, it too needs valid ticket cookies to use for checkout, just as your browser does if you cop manually. Your browser can generate valid ticket cookies quite easily -- and this is by design. Supreme only wants people using browsers to checkout. However, SwiftSole is a bot not a browser so it is much more difficult and in fact is becoming increasingly difficult to generate valid ticket cookies outside of a browser.