I don't know how the Itch app works, but that's the standard error message you get if you try to run a Unity webgl game outside the context of a web host.
There is a downloadable Zip file
that contains an exe version of the game
The itch app loads web pages from itch.io, but interacts with them differently than a standard web browser does. The app has an Install button that tries to find and install compatible games from the web page, and when I used it on this page it installed the “Web.zip” package. When I try to directly click the Download button in the web page (instead of the app’s own button) to get “Hollowspire.v.1.0.zip” it says “No compatible downloads found.” This is, apparently, because the download hasn’t been explicitly flagged as being for Windows.