what i'm supposed to do?

Contact the giveaway creator.

Ask the creator of the giveaway to give you a working key. If he/she does not do so, mark the giveaway as "not received" after one week.

Tell the giveaway creator that the key shows that it has already been used, he should then provide a replacement key or you can allow him to delete the giveaway.

Alternatively if he doesn't give a key and you don't want him to delete the giveaway it will stay a bad mark on his profile at the very least XD