SMH. Spent all morning trying to upload a working WebGL build but kept getting errors that only appeared when uploaded to Itch. Finally got a build that worked on Itch but sadly I can't update my game submission. Tis a sad moment. Next time I'll have to start submitting earlier.

So I've taken a look at a couple games now and it seems like other people are having this error. I think perhaps it is happening to WebGL builds from newer versions of Unity (personally I am using 2020.1.6f1).  When you build and run the game it works perfectly fine. However, when you upload the game to Itch.io it will never finish loading. I was able to fix this error by going to Project Settings -> Player -> Publishing Settings  and setting 'compression format' to 'Disabled'. I hope this helps someone upload their game to Itch (although sadly it won't be fixed in time for judging).