If you can't give out refunds then you should just give free steam keys to people who've already purchased it. I would highly suggest you do that unless you want to ruin your reputation and never buy anything you make in the future. Don't be short sighted and greedy.
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If you want the game on steam you have to buy it.
You can have the same game but without the steam integration by downloading the last file that was upload 12 days ago, that version is exactly the same game that is on steam but without some features that can only be available on steam, all the files are free for the people who bought the game here.
(Edit) Here you can read more about it: https://nachogames.itch.io/thats-not-my-neighbor/devlog/916937/remake-20-version..
Also I already explained why is not possible to give a steam key to everybody, but in resume, the number of keys required far exceeds the amount allowed by Steam (by a lot), so instead the game was upload here without the steam integration and is only available for the people who bought the first game.