I have been intermittently trying to get an answer to this question for the last two years or so and still haven't found a clear answer in the forums.
Version: 1.3.8 (if 1.3.9 will solve this issue, I will upgrade this very second)
I cannot restrict discount coupons to a specific category.
I changed the Master Category ID of all applicable products to the category I want the coupon to apply to but they are by necessity still linked to other categories.
Restriction settings I have on the coupon:
Top - deny
Category x - allow
When I place a product that should have the coupon code applied in my cart and try to redeem the coupon, I get the following message:
"This coupon code is not valid for any product currently in your cart."
I cannot add these products one by one in the restrictions. There are over 100 products in the category.