Error: You need to include some adapter that implements __karma__.start method

It does seem like this is a very general error, however in my case the problem was either that I didn't run karma start from the correct folder, or that I didn't restart it after changing the configuration.

I'll leave this question open and hopefully it can become a resource for others who experience this error message.

If you name your karma config file karma.conf.js, you can simply type karma start.

Otherwise specify the filename karma start karmafile.js

(I was in the right directory, but was not specifying a file name.)