Mistake?

https://www.steamgifts.com/support/tickets/new (delete giveaway)
Make sure to read: https://www.steamgifts.com/about/guidelines to be up to date with all the rules. While you're at it here's FAQ
Welcome to SG and enjoy your stay here :}

You can delete giveaway anytime for any reason while it is still going

Mistakes happen and what’s important is you are trying to fix it. :)

Open you giveaway: https://www.steamgifts.com/giveaway/zYqVW/sex-game-naughty-couple-episode-9

Click on Delete giveaway on the left side
Pick accident as a reason (doesn't matter)
The system will automatically, instantly delete your giveaway
Done.

Only works like this if it's still ongoing, you have 5 hours.

You can let the winner know that you no longer have a key for the game and let them mark it as "not received" or you can explain what happened to the winner and ask them if you can delete the giveaway. If they agree to delete the giveaway, you can open the giveaway page and click the button on the left side that says "Delete Giveaway". On the page it opens, you can select the reason "Did not understand how the site works" and give support a short explanation of what happened.

Edit: I didn't realize that the giveaway is still running. Because it is still running, you can delete it immediately without getting permission from the winner since there is no winner yet.

Delete the Giveaway before it ends.