flex auto 0 1是display的无效属性值。浏览器的文档检查器将向您显示此信息。您需要将display和flex属性中的这些值分隔开。
我猜想您也需要flex-direction: column，但我将把它留给您。

body {
  margin: 0;
  display: flex;
  flex: auto 0 1;
  justify-content: center;
  height: 100vh;
}

<form class="form" id="form"></form>
<h4 class="score" id="score">score :0</h4>
<h1 id="Question">What is 1 multiply by 1?</h1>
<input type="text" class="input" id="input" placeholder="Enter your answer" autofocus autocomplete="off">
<button type="submit" class="btn">submit</button>

关键是把它 Package 在一个容器中，使它成为内联块，然后使用flex将它居中：

body {
  margin: 0;
  display: flex ;
  justify-content:center;
  align-items:center;
  height: 100vh;

}
.container{
  display:inline-block;
}

<div class="container">
    <form class="form" id="form"></form>
   <h4 class="score" id="score">score :0</h4>
  <h1 id="Question">What is 1 multiply by 1?</h1>
  <input type="text" class="input" id="input" 
   placeholder="Enter your answer" autofocus        autocomplete="off">
   <button type="submit" class="btn">submit</button>
</div>