Go to the itch page settings and disable "Fullscreen button". That adds the little transparent button onto the iframe of the game, but since this is a Unity WebGL game with the default WebGL export framing, it already gets that nice little opaque blue button, which gets hidden underneath the transparent iframe fullscreen button. The Unity WebGL one is better, properly fullscreening and maximizing the game, whereas the itch-provided iframe button just removes the rest of the HTML page and centers the WebGL iframe. You shouldn't ever really have both, and they overlap, making it really annoying when you're trying to click the better Unity-provided blue fullscreen button hidden underneath.