It's the game itself. The demo works fine, but when I try the full version, it won't start.
Very odd - it's a free standing .exe so shouldn't install anything - you need to extract the whole zip file and put the folder wherever you want to store the game, don't run the exe out of the zip file.
If that's not the issue, the only 'installing' i can think of.. maybe your DirectX 12 is out of date? maybe try https://support.microsoft.com/en-au/topic/how-to-install-the-latest-version-of-d...
Aw - I did fail to mention the official Itch app - https://itch.io/app - if you aren't using it, try it that way, i think it might be your best chance.
If that doesn't work, lastly, if you don't play games often etc, the other thing that might be out of date on your computer could be https://aka.ms/vs/17/release/vc_redist.x64.exe (from https://learn.microsoft.com/en-us/cpp/windows/latest-supported-vc-redist?view=ms...) - outside of that, antivirus or soemthing with your computer set-up, so try it on a different Windows 10/11 machine.