I ordered off grubhub a few hours ago. Had no issues entering the promo code and got free delivery.
Maybe instead of the app, just go to the website directly. Make sure the order is $15 or more before tax, tip, and delivery fee. If this isn't working and you suspect it might be because you have grubhub+, you could try a new account. Other than that, idk. I hope you guys can resolve this issue.