bootstrap 4 and laravel 5.6 not showing error under input field

I am using laravel 5.6 and have the follwing in blade file:

<form method="POST" action="/review">
                    @csrf
                    

                    <input type="hidden" name="book_id" value="">
                    <div class="form-group">
                      <label for="nickname">nickname:</label>
                      <input type="text" name="nickname" class="form-control " id="nickname">
                      @if($errors->has('nickname'))
                      <div class="invalid-feedback" role="alert">
                            <strong></strong>
                      </div>
                      @endif
                    </div>
                    <div class="form-group">
                      <label for="review">comment:</label>
                      <textarea name="review" class="form-control"></textarea>
                      @if($errors->has('review'))
                      <span class="invalid-feedback" role="alert">
                            <strong></strong>
                      </span>
                      @endif
                    </div>

                    <button type="submit" class="btn btn-primary">Submit</button>
                  </form>

as you see in the third line I added just to make sure it has output and it does but just not showing the output under the input field. how to solve this? why it happened?

via Chebli Mohamed