Having problem with uncss

I an running the zurb template for foundation-sites. I upgraded to the newest version and everything was working fine. I then uncommented the line in gulpfile.babel.js that activates uncss in production. When I ran a production build I got an error that said it couldn’t find the file
but app.js is located in /src/assets/js/app.js . Any idea on what the problem is?

I found a typo in one of my links that was causing the problem.

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