You’ll have to look at the full error message (including the stack trace) and potentially the network tab on the browser console to see what’s going on.
For example, the Failed to fetch error likely includes the URL or path it’s trying to fetch. If it’s not pointing to a file that you have provided in your project then you have a bug in your code.
The common pitfalls article I linked talks about using relative paths over absolute paths, and talks about some of the common errors people have when they don’t correct distribute their HTML game.
Hope that helps