When you bought the game, you should be able to download it off the the itch io page
At the top of the store page, click the downward arrow next to your username, then click “My library”.
There’s a section called “Things you own”, and if the purchase went through, the game should appear with a button that says “Download”.
Click that, then the “Download” button next to “Buckshot Roulette (Windows)” (if your computer is running on Windows).