As is the case with any other missions in the game, you need to start completing the task after receiving it, and not before. This means that you need to open these chests after you’ve received this very task. What then is the solution? After visiting the bunker, a timer will appear on the global map. After the specified period of time, zombies, chests and loot will return to the bunker, and you’ll be able to once again exchange your coupons, thereby completing the quest!