Navigate to https://www.redbull....al-contact-form
Fill in your info and email.
in the message box complain about how your redbull was flat / gross...
Be sure you tell them that you buy redbull often and that its never had this foul taste and no carbonation.
If they ask you for info on your pack / can of redbull just say you threw it away because it was defective.. or go to walmart and take a picture of the numbers on packs of redbull or cans.
Redbull will offer to ship you a manufacturers coupon for free product depending what you told them (how much was defective, etc)