How to save dynamic select list value into config table in Drupal 8?
There is some problem in the submit handler.

Replace

$this-&gt;config('test.settings')
 -&gt;set('test_' . $content_type-&gt;get('type') . '_field', $form_state-&gt;getValue('test', 'test_' . $content_type-&gt;get('type') . '_field'))
 -&gt;save();


line with

$this-&gt;config('test.settings')
  -&gt;set('test_' . $content_type-&gt;get('type') . '_field', $form_state-&gt;getValues('test_' . $content_type-&gt;get('type') . '_field'))
  -&gt;save();


Hope that this helps you.