Sorry to hear that. Have you emailed itch.io support as they are the ones who can help with that?
I did, but haven't heard back from them yet. Just posted here cause in support's FAQ page doesn't make clear who's responsible for that:
"There are a few reasons why a Steam key may not appear. If the developer has run out of keys for their page, then you will not be able to claim a Steam key. When this happens, the developer is notified via an automated system to refill the keys."
Does that mean that you haven't been notified by this automated system yet?