Hi.
I have made several games in Unity and comilated them for WebGL, uploaded them to itch and all them work fine. Compression format can not be a reason of your problem (I use Brotly, it is the best).
For the first, try to use the "Build and Run" feature - Unity will build the project and will run it locally in your browser. If the game will work fine, so the project has been built right way. Next you have to pack it into ZIP without the root folder, so the index.html has to be in the root of archive (it is important!).
Upload that ZIP to the site and all will work good.