First check you have entered your linked credit card details correctly. You are also required to have sufficient funds on the credit card to cover the first payment. You also need to pass Afterpay's own internal screening process before they will extend you credit. Any previous Afterpay defaults and bad credit may affect your eligibility. Please contact Afterpay's customer service team on 1300 100 729 and they can assist you in completing your order.
See Afterpay's help center here