Wordpress shortcode - include_once cant find file in "includes" folder? php, mysql

Question

The plugin "Shortcoder" in Wordpress doesent fint the included file? Anyone that knows how to find where the page with the embedded shortcode is located in the folder hirarki? I have tested all from "/includes/book.." to "../../../includes/" but i get "page not find!" error message.

probably something with this part i belive. '../../../includes/booking_available_dates.php'

<?PHP
  require_once('../../../includes/booking_available_dates.php');
?>

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css"> 
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css">

<form name='date_registration' action='booking_confirm_view.php' method='POST'>
   
  <div class="form-group row">
    <label for="select" class="col-2 col-form-label">H?mta f?rsta lediga tid f?r valt datum</label> 
    <div class="col-10">
      <select id="date_input" name="date_input" class="custom-select">
      <?php foreach ($dates as $row): ?>
      <option><?=$row["date_booking_customer"]?></option>
      <?php endforeach ?>
      </select>
    </div>
  </div> 

  <div class="form-group row">
    <div class="offset-4 col-8">
      <button name="submit" type="submit" class="btn btn-primary">H?mta tid!</button>
    </div>
  </div>
</form>

Answer 1

等待大神答复