Trying from the Steam client (as opposed to the Steam web page) returned the same results. I did click on the link to recover the account; this won't tell me who the owner of the key is, but as I recall it sends a note to the person to whom the key was redeemed.

I understand that a replacement key isn't available. I'll have to mark the win as not received, as that's the process, but I believe you've acted in good faith. I think the only direct effect of a key being marked as not received is that you don't get that giveaway slot back, in terms of the maximum number of simultaneous giveaways you can create, but you can ask support to increase your limit if that becomes an issue. Thank you again for making the giveaway, even though the key did not redeem for me - best wishes & good luck.