I double-checked and the key is identical. There's no error in the key because it's a copy-and-paste job.

Maybe it doesn't work in your region. It seems you're in SEA region according to your Steam?

I revealed the key in my Humble a month ago and copied it outside to avoid seeing the Ran out of stock message later. No region-lock warning showed for me back then.

Apologies for the confusion.

May you allow the win to be re-rolled so someone else can enjoy the game, please? Thank you for your understanding :)