I believe someone else had a similar issue where their machine was telling them it couldn't find the file.
Their advice is to uninstall and remove the demo if that is installed as well as try to download directly from the website and not the itch app.
Beyond that, I am not sure what advice to give. Unfortunately download issues are a little outside my wheelhouse if the game build itself is working because I can't say for sure what is happening between your computer, Itch, and the game build itself.