I think I had to export several times to get something that would work when uploaded (and maybe I could check the names of the files it generates, but running the HTML5 version from a file on a computer doesn't seem to work, even if that same web version will work fine when uploaded on itch.io. Fortunately I uploaded the HTML5 version like an hour before the deadline, so I could just regenerate and retry for every time it didn't work. I'm feeling maybe the Windows and Linux builds of my game that I uploaded don't work though. Maybe I should have tested those before uploading.
You're welcome!