1123981:You can try e-mailing entertainment@upperdeck.com for help. Technically once you redeem the code on Blizzard's website Upper Deck is no longer involved, but it seems that Blizzard is pretty unresponsive about loot issues.
BlueRider has the correct advice here. While our ability to help is marginal once you have redeemed the code on Blizzard's site for an in-game code, we are familiar with many of the typical problems that users face when trying to pick up their item from Landro.
Contact us at the above e-mail address and we'll see what we can do. In your situation, I would first ask if you are absolutely certain the code you were given was for a Tabard of the Void and not one of the other Tabards. Also, the 'could not validate code' message is consistent with trying to use a code that was redeemed on a different server. You might just try it again and see what server it is on via Blizzard's redemption page.
-Patrick
Patrick Swift
OP Support Team Manager
Upper Deck Entertainment
70 Undead Warlock
29 Troll Hunter