Hi guys! I bought a game called "Dwarf fortress" and it has a bundled steam key when I purchased it. However after purchase I don't know where it is.
I tried looking on the email, download page and inside the zip file but I can't find it.
From what you describe, I would assume that the game ran out of Steam Keys.
You can read more about it here: https://itch.io/docs/buying/already-bought
It's at the end of the article.
If you have no way of contacting the developer (maybe on Twitter, or via a support mail they have added to their itch.io account), you probably have no option than to wait for support to give you an answer.
The support mail should show up here if available: https://itch.io/support