Thank you for reaching out, and I apologize for the confusion regarding the keys.
Currently, the Early Access build is only available on Itch.io. While we do have a Steam page, the game has not been officially released on Steam yet. The key you received on your download page is specifically for the Itch.io version, not for Steam. (When the Steam version is ready, you can get a Steam key on your download page to redeem the game on Steam)
I have just posted a new Devlog with more details regarding this and our upcoming plans. Please check it out for a clearer guide on how to access the game.
https://goldenunicorn.itch.io/boys-in-the-dark/devlog/1507296/early-access-is-li...
Thank you for your patience and for supporting the project!