If a coupon isn’t working, try these things:
>> Be logged in to the site FIRST.
(Coupons are often assigned to a specific email.)
>> Be sure to click to accept the code, and verify the price has been adjusted before continuing to the checkout page.
>> For Gift Certificates (for example, the Master Course Gift Certificate) the one purchasing should have sent the email address/account in the checkout notes or as a reply to the purchase invoice email.