Coupons can only be applied to new Reels and cannot be applied to a Reel that is already being saved towards or transferred to any other existing Reels.
When Adding a Coupon from Reel's site:
STEP 1: Search for your item or click on Start a Reel.
STEP 2: Select "Apply Promo Code", enter the promo code and click "Apply". If the coupon is valid, the coupon amount will be applied to your Reel.
STEP 3: After the coupon has successfully been added to your Reel, click "Start Saving" to confirm your Reel and you are all set!