When you use ClassicEditor form '@ckeditor/ckeditor5-build-classic', it realy have Mention plugin, so you don't need install '@ckeditor/ckeditor5-mention'

     import { CKEditor } from '@ckeditor/ckeditor5-react'
     import ClassicEditor from '@ckeditor/ckeditor5-build-classic'
    
     const CreatePage = () => {
     return (
     <div>
       <h2>CKEditor with Mentions</h2>
       <CKEditor
         editor={ClassicEditor}
         config={{
           mention: {
             feeds: [
               {
                  marker: "@",
                  feed: ["Alice", "Bob", "Charlie"],
                },
              ],
           },
        }}
      />
     </div>
     )
    }
    
    export default CreatePage`