Maybe you do not have Chrome installed and you were trying to make this work with IE or Firefox. Unfortunately, when you configured IE or Firefox instead of Chrome, you were met with an error upon karma start. The error you see is Cannot load browser "Firefox": it is not registered! Perhaps you are missing some plugin?. Well, perhaps you are missing a plugin indeed, so let's install it:
npm install karma-firefox-launcher --save-dev
Now, you can change browsers in your Karma configuration to Firefox. If you run Karma now, you will see it uses Firefox instead of Chrome. You may need to restart your console before this works.
You can install launchers for IE, Edge, and other browsers as well:
npm install karma-ie-launcher --save-dev ...