First of all, please check that the discount code has not already been used on another order or has not expired.
However, it is not possible to use a discount code that exceeds the total value of the order.
With regard to promotional discount codes, their use may be linked to the purchase of a product and are not cumulative. They may also require a minimum order amount.
Lastly, did you remember to click on the little arrow to validate the discount code?