I didnt like the way I was treated, i asked if i could use the coupon before hand, she said yes, and then when i was finished i had to pay and extra 6 dollars because she wouldnt let me use it? how does that work?

Yes, a coupon can be run through a point of sale system after the fact. If your coupon wasn't expired, I'm assuming the server just didn't want to go back and fix their mistake. You should've asked for the reason it wasn't being accepted. And if necessary, you could've politely asked to see the shift manager to discuss it.

I am not real sure but if your coupon is expired they cannot accept it