If you're trying to play through the itch app, that doesn't work with the web version, you need to play it on the game page in a browser, ideally Chrome/ a chromium based browser because Firefox can cause issues. If that's not it, make sure graphics acceleration is on in your browser settings.
However, I also just released a downloadable build now as part of the custom map support update, which should work if you can't get the web version to work