CKeditor doesn't work in controller response within a default theme
A form usually contains all libraries necessary, but you shouldn't render the form at this point in the controller.

Put it unrendered in the template variable

return [
  '#theme' =&gt; 'customize_user_form',
  '#form' =&gt; $form,
];


and when you print the form in the template {{ form }} the libraries should be attached automatically to the page.