Call to undefined function
I'm assuming creative_customize_comment_form() is a custom form function, so it probably exists in either:


A file inside a module that's not enabled, so Drupal can't find it
An include file in the module that's not automagically included during the page build.


The solution to the first is obviously to enable the module.

The solution to the second is to Make sure the appropriate file is included in the page build, using module_load_include(), e.g.

module_load_include('inc', 'creative_customize', 'creative_customize.pages');
// Now call drupal_get_form()...


Without more information I can't think of any other reason (other than the function name itself being incorrect) that drupal_get_form() would fail.

Source: https://api.drupal.org/api/drupal/includes!module.inc/function/module_load_include/7