When you say "Coupon is individual", do you mean that there is one coupon id per recipient in the targeted audience? Recipient A receives coupon code A while recipient B receives coupon code B. Or is it the same coupon code across the whole delivery?
Do you have a record in the nms:coupon entity that is linked to the Delivery via the coupon-id field?
The syntax to add the coupon code might be <%= message.delivery.coupon.code %>