Ajax not working in text field
You need to declare on what object (or class if static) to call the callback, like

'callback' =&gt; [get_called_class(), 'settingsAjax']


or 

'#ajax' =&gt; [
            'callback' =&gt; [get_class($this), 'addMoreAjax'],
            'wrapper' =&gt; $wrapper_id,
            'effect' =&gt; 'fade',
          ],


or at least

'callback' =&gt; '::settingsAjax'


depending on your code.