On Steam, the offer is valid until November 6 at 19:00 CET.
But on Humble Bundle you still have 5 days to download (until November 9). You will get a code to download it on Steam. The regular price is £3.99 (4.99 euros), so it's not like you need to break the bank, if you happen to miss this offer.
"Subscribe to the Humble Bundle newsletter and we'll email your free game to you. Hurry, this offer is available only while supplies last, or until November 9, 2020 at 10:00:00 AM PST!"
You can read our review about the Kingdom right here.