It is unfortunately not possible to apply a voucher to a purchase after the order is complete. Vouchers must be redeemed during the order process.
Please enter the relevant voucher code in the final order step "Overview" and activate it by clicking on the button "Redeem". You will see that the voucher has been credited successfully in the overview of the order total.
If the order has not yet been processed, you have the option to cancel and place another order with the applicable voucher. You can find more information about this under Canceling an order or changing an order retrospectively.