To make a new coupon code, simply navigate to the course that you want to sell, and click on the Sell Settings button.
Next, click Add a Coupon Code and enter the code you want used for the coupon. Next enter the discount value and specify whether the discount is in your platform's currency or a percentage. Hit Add Coupon Code when finished.
Once you're done, click Update Settings 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.