That's a new error, and it's because I changed some internals[1]. For now, you'll have to open the link[2] in a new tab and do the steps there.
[1] For about a week (and at the time of posting) the app was on 90s.dev/os/ to avoid this exact CORS error. But I got tired of how it forced me to name my github repos or pay for my own server, so I moved it back to os.90s.dev, which has the one downside of the error you ran into.