Hello! When you can't find the key there's two things to check first:
1. Is your itchio account connected to your Steam account? You can connect them in your settings (the "connected account" menu, then use the Steam button), and they need to be connected to redeem a key.
2. Be sure you're checking the right place. At the top of the OBSCURA page should be a grey bar informing you that you've already purchased the game, with a link to the download page. The Steam key redemption button can be found below the other download options.
If you've cleared both of those hurdles, then I'm less certain about what could be going on. There are plenty of keys left to claim, at least.