Being logged in with student status blocks entering a coupon code as discounts cannot be combined. To take advantage of a coupon, log out of your customer account and empty the shopping cart. Then (re)add the product you’d like to buy to the cart and enter the coupon code.
Please understand that it is not possible to upgrade an existing license purchased as a student or educator using a coupon code. The standard students & educators discount is automatically applied in this case.