Masalahnya terletak pada fungsi addTopic anda:

e.target.value sentiasa tidak ditentukan

Untuk mengakses data yang anda perlukan untuk melakukan ini:

const addTopic = (e) => {
    e.preventDefault()

    const myData = {
        title: e.target.title.value,
        message: e.target.message.value
    }
    
    setTopic(prev => [...prev, myData])        

}

Cara mudah ialah melakukan ini.

Anda perlu menggunakan onChange input untuk mendapatkan nilai yang anda perolehi.

Contoh pautan: https://stackblitz.com/edit/react-8r9f8l?file=src%2FApp.js

import React, { useState } from 'react';

const ForumTopic = [
  {
    title: 'First Post',
    messages: 'test',
    author: 'Dagger',
    count: 1,
    date: '02/16',
  },
];

export default function App() {
  const [topic, setTopic] = useState(ForumTopic);
  const [inputObj, setInputObj] = useState({
    title: '',
    messages: '',
    author: 'Dagger',
    count: 1,
    date: '02/16',
  });

  const handleChange = (event) => {
    setInputObj((curr) => ({
      ...curr,
      [event.target.name]: event.target.value,
    }));
  };

  const addTopic = (e) => {
    e.preventDefault();
    setTopic([...topic, inputObj]);
  };

  return (
    <>
      <form onSubmit={addTopic}>
        <label htmlFor="title">
          创建一个主题标题
          <input
            id="title"
            name="title"
            value={inputObj.title}
            onChange={handleChange}
          ></input>
        </label>
        <label htmlFor="message">
          写下您的消息
          <textarea
            id="message"
            name="messages"
            value={inputObj.messages}
            onChange={handleChange}
          ></textarea>
        </label>
        <label htmlFor="author">
          <input id="author" name="author" defaultValue="Dagger" hidden></input>
        </label>
        <label htmlFor="count">
          <input id="count" name="count" defaultValue="1" hidden></input>
        </label>
        <label htmlFor="date">
          <input id="date" name="date" defaultValue="02/16/2023" hidden></input>
        </label>
        <button type="submit">发布新消息</button>
      </form>
      {topic.map((item) => {
        return (
          <>
            <p>{item.title}</p>
            <p>{item.messages}</p>
            <p>{item.author}</p>
            <p>{item.count}</p>
            <p>{item.date}</p>
            <span>------------</span>
          </>
        );
      })}
    </>
  );
}