ng build --prod will create build artifacts in the /dist folder - you have to put those files on a different web server (i.e. you don't use ng serve after that).
If you just want to run your app using ng serve but using production mode,
then use:
ng serve --prod
Edit
I don't seem to be doing a good job of explaining this. Your request is to "enable production mode". You have two ways of doing that, depending on what you're trying to do.
If you want to use the angular-cli development server to run your app in production mode as a means of testing to see if production mode works properly, then use:
ng serve --prod
If, however, you are ready to deploy your app to a production web server, then it's a two step process:
generate the production-ready version of your app using: ng build --prod
copy the output of the /dist folder onto your production web server
Note that I don't do both - i use either ng build or ng serve , depending on what I'm trying to accomplish.