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

After you have assigned users to your coupon, click Publish/Update.
The Coupons & Discounts for LearnDash plugin will now feature a coupon assigned to a specific user.