Karma not running unit tests due to "No captured browser" message

I just had the same issue in a different setup (Linux, QUnit, Firefox), though. The problem disappeared after I killed all karma processes and did a fresh karma start.

I had a face palm moment. karma start needs to be run in the same directory as your karma.conf.js file.

This error can mean that the browser can't find the server. Check if you can reach the server at the URL it mentions. It could be a misconfigured port number, or even (as it was in my case), localhost being misconfigured. I suppose it could be the server is not running, or any of a dozen other problems.

Check if you can reach the server manually. If you can't, fix that.