After following the steps outlined in How to Create a Coupon for LearnDash, click the toggle next to Course Restriction to enable the function.
You will be prompted with a drop-down menu, Allow Courses, to select which courses you would like the coupon to apply to.

After you have assigned courses to your coupon, click Publish/Update.
The Coupons & Discounts for LearnDash plugin will now restrict coupons to specific courses.