It could be many things, for example, that you have the game with opengl 3.0, but the browser only supports 2.x.
Or that you marked an incorrect option at some point.
What I advise you is to create a new project in godot, something very, very simple, for example, displaying just one image.
- Export to HTML (remember to create a clean folder to export)
- Rename ONLY the .html file to index.html
- ZIP the folder where you exported the game.
- When you upload it to Itch.io, remember to check the shared buffer option.