스파르타 피디아의 포스트박스가 적용되지 않네요.

<div class="mypost">

       <div class="form-floating mb-3">
          <input type="email" class="form-control" id="floatingInput" placeholder="name@example.com">
          <label for="floatingInput">영화 URL</label>
       </div>

      <div class="input-group mb-3">
          <label class="input-group-text" for="inputGroupSelect01">별점</label>
          <select class="form-select" id="inputGroupSelect01">
            <option selected>선택하기</option>
            <option value="1">⭐</option>
            <option value="2">⭐⭐</option>
            <option value="3">⭐⭐⭐</option>
            <option value="4">⭐⭐⭐⭐</option>
            <option value="5">⭐⭐⭐⭐⭐</option>
          </select>
      </div>

      <div class="form-floating">
          <textarea class="form-control" placeholder="Leave a comment here" id="floatingTextarea2" style="height: 100px"></textarea>
          <label for="floatingTextarea2">코멘트</label>
      </div>

      <div class="btn">
          <button type="button" class="btn btn-dark">기록하기</button>
          <button type="button" class="btn btn-outline-dark">닫기</button>
      </div>
</div>

포스트박스가 500px 가운데로 오지가 않네요.

작성한 코드 및 에러 메세지

.mypost{

  max-width: 500px;
    width: 95%;
  margin: 20px auto 0px auto;

  box-shadow: 0px 0px 3px 0px gray;
  padding: 20px;
}