Student discounts are only available for CMS site plans. If you are attempting to use the discount on a Workspace plan it will not work. Create a site and then add your discount code when purchasing a CMS site plan.
The email address you applied for the discount with must match the email address used for your Webflow account.
When applying for a student discount, make sure you select the appropriate option for the question, "Do you have a school associated email address?" If you have a free email account such as a gmail.com or outlook.com, make sure you select No. If your promo code is not working, try applying again selecting the appropriate option for that question.