If you chose the store pickup option, then this page asks for the details of the individual picking the order up. This individual must present the coupon in person to the cashier upon payment and it will be applied to your order. You can not enter coupons on Walmart's website. You must present them in person after having chosen store pickup.
Enter this information and then choose Cash payment. Walmart will then reserve your product pending your arrival at the specified store. When checking out you can present your coupon.
To use your Gift Card, you can choose that option now.