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?

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