Hi Emi!
It doesn't come with a Steam key - and I am sorry that it isn't extracting for you! This being said, if you want to email me proof you have bought the game on Itch (cottoncandycyanide(at)gmail.com), I will send you a Steam key! :) You shouldn't have to buy the game twice when one isn't working~ (also, if you could send a screenshot of what is happening when trying to extract so I can try to troubleshoot for future players, I would super appreciate that too! :D)
<3 MoltenCherry