#ajax form_set_error
Your code needs to be adjusted to something like:

function tubitekuz_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  switch($form_id) {
    case 'webform_client_form_97':
      global $user;
      if ($user-&gt;uid &gt; 0) {
        $form['submitted']['kullanici_adi']['#access'] = FALSE;
      }
      $form['submitted']['kullanici_adi'] += array(
        '#prefix' =&gt; '&lt;div id="kullanici-adi-container"&gt;',
        '#ajax' =&gt; array(
          'callback' =&gt; 'kullanici_adi_form_login',
          'wrapper' =&gt; 'kullanici-adi-container',
        ),
        '#suffix' =&gt; '&lt;/div&gt;',
      );

      // Check the form state to see if an e-mail address has been specified.
      if (!empty($form_state['values']['kullanici_adi'])) {
        $mail = trim($form_state['values']['kullanici_adi']);

        if ($account = user_load_by_mail($mail)) {
          form_set_error('submitted][kullanici_adi', 'Kullanıcı adı kayıtlı, lütfen başka bir kullanıcı adı seçin');
        }
      }
      break;
  }
}

function kullanici_adi_form_login($form, $form_state) {
  return $form['submitted']['kullanici_adi'];
}


You will probably have to play around with some of the arrays, etc. to get the correct values, etc.