itch.io

There are few reasons why it may happen (AI answer):

1. You're not actually running it from itch page
   A lot of people download the HTML build and double-click index.html (file://) or run it from some random place. That cannot send the required headers → boom, this exact error. 

2. In-app / embedded browser weirdness
   If he opened it inside Discord/Reddit/itch desktop app/in-app browser, those environments sometimes don’t apply/allow the isolation correctly, even though “normal Chrome/Firefox tab” works.

3. Privacy tools / corporate network stripping headers
   Some extensions (ad blockers, privacy script injectors) or corporate proxies can break cross-origin isolation by altering requests/headers or injecting scripts.

4. Browser/version mismatch (often Safari/iOS or outdated browsers)
   Cross-origin isolation + SharedArrayBuffer support isn’t uniformly reliable everywhere; older Safari/iOS setups are common pain points.

Try these:

• “Please open the game directly on itch in a normal desktop browser tab (latest Chrome/Firefox/Edge), not inside Discord/itch app.”

• “Try Incognito with extensions disabled.”

• “Make sure you didn’t download the HTML and open index.html locally.”

• “Tell me your browser + version + OS (and whether you opened it inside an app).”