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ask for a new key i guess, other than that never click accept unless you get it

1. Write a comment in the giveaway.
2. Wait 7 days.
3. If the creator doesn't send you a good code, mark as not received because you haven't received a working code.

If you mark a game you didn't get as Received, then it will certainly have a negative impact on your account.

From the FAQ (https://www.steamgifts.com/about/faq):
"When should I mark a gift I won as "Received" or "Not Received"?
You should only mark a gift as "Received" when you have both (i) received the exact game described in the giveaway, and (ii) activated that game on the Steam account you used during registration. In all other instances you should mark the gift as "Not Received" one week after the giveaway has ended."