If you're unable to retrieve the offer, the voucher redemption server may be down temporarily, or you might not be connected to the internet. You can try again later by coming back to the Google Drive™ application and selecting the “Product Tour” menu item.
If you receive the error message: "The offer has already been redeemed", it may mean someone has already redeemed the offer on your phone and you will not be able to redeem this offer twice. Alternatively, it may also mean your Google Account has already claimed this offer and you will not be able to redeem the offer a second time.