This error is returned when no cost is assigned for the Cash On Delivery payment method (COD), even though this payment method is selected among the available one. To solve this problem, you need to find out the module configuration (first tab) and assign an additional cost (or “0” if you do not want to bid) to the COD service. If you want to specify decimal values, use the point as a decimal separator (Eg. 3.50 / 3.5 €).