Open up the browser dev tools and debug your website.
Extensions are just websites so normal website debugging applies.
You declared a 400 but you didn’t state what was 400ing
I suspect your CSS/JS is whats 400-ing as you used an aboluste link/reference to the JS/CSS in your HTML instead of a relative one as extensions upload to a sub folder.
Or you didn’t state the html to load was index.html in the dev console.