I double checked the key and it's definitely the right one. It also hasn't been redeemed by anyone else. Have you tried both c&p, as well as manually entering the key just to make sure there isn't a transcription error? Also, after googling if there was a known issue with that game from that bundle, may I ask if you're activating from Russia or Belarus? I only just found out that users from these countries are excluded from redeeming keys from the Support Ukraine bundle, since I wasn't aware I didn't enter a region restriction when I listed the giveaway. My apologies for that. That's the info here
https://www.steamgifts.com/discussion/RTgYQ/humble-bundle-stand-with-ukraine-bundle-back4blood-restocked-for-eu-iron-danger-added/search?page=5
If that should be the case, would you drop me a comment so I can request to have a new winner drawn?
I also checked if I could afford to buy and gift another copy on Steam to make up for this key not working, but tbh it's not in my budget right now.