Uncaught TypeError: Невозможно прочитать свойство «_setter» undefined

You have missed a column for Extension

 <script>
  $(function() {
      $("#people").DataTable({
        "iDisplayLength": 25,
        "processing": true,
        "serverSide": true,
        "ajax": 'http://example.com/datatables-people',
        "columns": [
          { data: 'firstname', defaultContent: '' },
          { data: 'lastname', defaultContent: '' },
          { data: 'extension', defaultContent: '' },
          { data: 'userid', defaultContent: '' },
          { data: 'title', defaultContent: '' },
          { data: 'FK_department', defaultContent: '' },
          { data: 'pagerid', defaultContent: '' },
          { data: 'FK_authcode', defaultContent: '' },
          { data: 'roomnumber', defaultContent: '' },
          { data: 'pubroomnumber', defaultContent: '' }
        ]
      });
  });
</script>