It's because you are importing the plugin with the build ! In order to add plugins, you have to make a personnal build. Please read this page to know more about it : ckeditor offical documentation. They even have an official online builder that does all the work for you ! : ckeditor online builder Once you created it, you have to import the editor just as you have done before on line 2 but instead of writing from "@ckeditor/ckeditor5-build-classic" you have to write from "adress of the build folder of your personnal build".
I hope it helped you.