Poor service

I made an order on the steers app for their cheese burger promotion that comes with a free slushy, to my surprise when my delivery arrived there was no slushy. I called the branch to enquire & initially they were not aware of the promotion then after some back & forth and them trying to blame me for ordering the wrong thing they say they don't have the slushy in-store. Now how is that my problem that the app doesn't correspond with what's in store?

Im not happy at all that they're not even trying to make up for it, just "we're sorry for the misunderstanding"

Its not a misunderstanding, I ordered what they don't have so how am I being compensated??