If you need to cancel a card order (for example, before the card is activated), you have two options.
If you have 'Account Owner' or 'Admin' rights, go to your Cards tab, select the card you've just ordered, click on "Mark as lost / unused / damaged / stolen". Then, click on Order a new card if you need to order another one.
If you don't have 'Account Owner' or 'Admin' rights, ask them to do it for you. It's indeed not possible for a Requester to cancel their card if it's not activated.
You might receive a card, still, if you cancel the order too late. If this happens, destroy and ignore it.
If you're unsure of who your Account Owner or Admin is, reach out to your favorite support team at firstname.lastname@example.org 😉.