//submit button
$form['action'] = [
'#prefix' => '<div id = "ajax_submit">',
'#name' => 'ajax_action',
'#type' => 'button',
'#value' => $this->t('Submit'),
'#ajax' => [
'callback' => '::setMessage',
'wrapper' => 'scrollable',
'method' => 'replace',
'effect' => 'fade',
],
'#suffix' => '</div></div></div>',
];


//table element
    $form['table'] = array(
   '#prefix' => '<div id="scrollable" class = "col-md-8"><h4>View Data</h4>',
   '#type' => 'tableselect',
   '#header' => $header,
   '#options' => $options,
   '#disabled' => $disabled,
   '#attributes' => array('id' => 'sort-table'),
   '#empty' => t('Select criteria and search data'),
    );

//pager element
  $form['pager'] = array(
  '#type' => 'pager',
 '#suffix' => '</div>',
  );

 //function to obtain #options
    if ($form_state->getTriggeringElement()['#name'] == 'ajax_action') {
        $options = array();
        $header = [
            'subject_id' => t('Subject ID'),
            'ct_availability' => t('CT Availability'),
             'sex' => t('Sex'),
             'race' => t('Race'),
             'cardiovascular_disease' => t('Cardiovascular Disease'),
             'registration_age' => t('Registration_Age'),
               ];
        $options = $this->getOptions($form_state); //function to get #options
        $form['table']['#header'] = $header;
        $form['table']['#options'] = $options;
        }

public function setMessage(array $form, FormStateInterface $form_state) {
return (array($form['table'], $form['pager']));