If you're having trouble with your payment method, it's likely it is unsupported, and you will need to try adding a different one. See How do I add a new payment method?
My bank is not accepted as a default payment method for first installments.
Bank accounts cannot be used as default payment methods for first installments on new orders. You may set your bank account as the default payment method for scheduled installments. However, you must have a debit or credit card on file set as the first installment payment method on your account. You can change your default payment methods for the first and following scheduled installments by navigating to your Sezzle dashboard, selecting "Accounts," and then "Payment Methods."