First, make sure it has been installed - there should be a karma-html-reporter directory in node_modules in the directory from which you are running the tests. If not run:
npm install karma-html-reporter
Then confirm you have 'karma-html-reporter' in plugins in the conf, i.e:
plugins: [ 'karma-teamcity-reporter', 'karma-jasmine', 'karma-coverage', 'karma-chrome-launcher', 'karma-phantomjs-launcher', 'karma-html-reporter' ]