The "platform unsupported" message is because the itch.io version of the game doesn't have proper support. The devs have said, without promising a timeframe, that they'll be working on this, but then released a roadmap with no mention of it and it's been more than 3 months without any further word.
You should, on the download page for the game, have a Steam key. Obviously there's good reasons for some of us to avoid Steam (like the fact that it's been known to be causing harm to customers since 2019) but if you can use that, it's an option. Otherwise get a refund and repurchase the game through Epic.