uhhh, I just purchased this and there's no Steam key.
Have you tried downloading the game yet? You can find your key by looking at the project's download page. Here's an itch.io guide for that: https://itch.io/t/67604/how-to-get-a-steam-key-from-your-itchio-purchase
If this doesn't work, this is a problem with itch.io. You should contact them directly.