The POS doesn't see any items in the transaction that are discountable. Check your menu item definitions. There is typically a parameter that says "is this item discountable" or "item discount" etc. Check that as active and it may begin to work.
Also you may also have discount itemizers (IE:buckets) to program. Many POS have multiple discount levels IE" 1,2,3,4. It's there so that you can put all food into discount itemizer (bucket) one, all liquor in itemizer (bucket) two. Then when you program menu items and discount buttons you can separate discounts that effect food vs. liquor. IE: senior citizen discount button(food only) would be set up to see discount itemizer #1, Happy hour discount button would see discount itemizer #2 (liquor), Programming a 100% discount would see (use) itemizer 1&2 so that all F&B are discounted.
I think your issue is in each menu item, check to see if your items are set to allow discounts.