I'm not sure if you mean that the web build isn't working on itch.io's end or locally.
If you're trying to run it locally: in Godot, if you have exported as a web build, there should be an icon to run it in-browser next to the run scene button at the top of the engine window. Clicking it should open the game in the browser.
If you're trying to run it on itch.io: export the project as "runnable" to a folder, with the html file named "index.html" This is what itch.io looks for in build builds. Then zip everything in the folder under a single zip and upload that