Do not use the Itch.io launcher It might bug with it. Just download the game from the web version of Itch.
Viewing post in CustomHero (WIN/MAC/LINX)- Extra - comments
What you have brought is available on the page. You should have a red button where you can download the game that is substituting the [buy now] button.
What you have brought is a separate mini game that is inspired of the full game. The full game is different and need to be brought and downloaded on page indicated on the FULL GAME link.