Your code should work, something outside of it or the way you are rendering the component is probably the issue.

Here's a working example using a functional component:

CreateNewPost.js

import { useState } from "react";
import { createReactEditorJS } from "react-editor-js";
import { EDITOR_JS_TOOLS } from "./constants";

const ReactEditorJS = createReactEditorJS();

const CreateNewPost = () => {
  const [editorValue, setEditorValue] = useState({
    time: new Date().getTime(),
    blocks: [
      {
        type: "header",
        data: {
          text: "Testing title",
          level: 2
        }
      },
      {
        type: "paragraph",
        data: {
          text:
            "We have been working on this project more than three years. Several large media projects help us to test and debug the Editor, to make it's core more stable. At the same time we significantly improved the API. Now, it can be used to create any plugin for any task. Hope you enjoy. "
        }
      }
    ],
    version: "2.1.0"
  });

  return <ReactEditorJS tools={EDITOR_JS_TOOLS} data={editorValue} />;
};

export default CreateNewPost;

index.js

import React from "react";
import { createRoot } from "react-dom/client";
import CreateNewPost from "./CreateNewPost";

const container = document.getElementById("app");
const root = createRoot(container);
root.render(<CreateNewPost />);