The key doesn't activate, it appears it is region restricted:
"Sorry, but the following items are not available for purchase in this country. Your purchase has been cancelled". I am in the EU.
See also this comment in the Monthly thread: https://www.steamgifts.com/discussion/bKFpq/humble-bundle-april-2023-humble-choice-41-leaks-for-may#5aIiUic.

I suggest you retry a new giveaway putting the correct region restriction.

I am ok with deleting this giveaway.

Thanks anyway, and stay safe :)