To make a new coupon code, simply navigate to the course that you want to sell, and click on the Sell Settings tab. Next, hit the green Coupon Codes button.
Next, enter the code you want used for the coupon, and enter the discount value, either in your platform's currency or as a percentage. Hit Add Coupon Code when finished.
Once you're done, hit Update on your course to save the changes. Now, when your learners want to buy the course, there will be a Coupon Code field that they can fill out to receive a discount.