When going to SETTINGS > STOREFRONT, you will find the Discount Codes/Promos section.

You have the ability to turn ON or OFF the entire Promotional Code section. If this is turned OFF, none of your codes will work and there will not be a place for promotional codes to be entered on the eCommerce website.

Once you turn ON Discount Codes/Promos, this enables the Promotional Code section on the eCommerce website.