Allow an action to return a string in cakephp 4.2.4

Need Help
1

I have a template with one dependent select with another (affiliate with enterprise), I fill that dependent select with ajax and jquery, but for some reason the request gives me an error: Error: Controller actions can only return ResponseInterface instance or null. Got string instead.
How can I solve this issue?
This is the code of the action:

    public function add()
    {
		$this->loadModel('Observations');
        $affiliateCertificate = $this->AffiliateCertificates->newEmptyEntity();
		if($this->request->is('ajax')) {
			$this->autorender = false;
			$enterprise_id = $this->request->getData('enterprise_id');
			$affiliates  =  $this->AffiliateCertificates->Affiliates->find('list')->where(['enterprise_id' => $enterprise_id]);
			$html = '<option value="0">- Seleccione un afiliado- </option>\n';
			foreach($affiliates as $key => $value) {
				$html .= '<option value="' . $key . '">' . $value . '</option>\n';
			}
			return $html;
		}elseif($this->request->is('post')) {
			$discount_request_url = $this->request->getData('discount_request_url');
			$type = $discount_request_url->getClientMediaType();
			if($type == "image/gif" || $type == "image/jpeg" || $type == "image/x-png") {
				$affiliateCertificate = $this->AffiliateCertificates->patchEntity($affiliateCertificate, $this->request->getData());
				$affiliateCertificate->discount_request_url = DS . 'constancia_afiliacion' . DS . $discount_request_url->getClientFilename();
				$affiliateCertificate->discount_request = WWW_ROOT . 'constancia_afiliacion' . DS . $discount_request_url->getClientFilename();
				$affiliateCertificate->discount_request_filename = $discount_request_url->getClientFilename();
				if ($this->AffiliateCertificates->save($affiliateCertificate)) {
					$discount_request_url->moveTo(WWW_ROOT . 'constancia_afiliacion' . DS . $discount_request_url->getClientFilename());
					$observations = $this->Observations->newEmptyEntity();
					$user = $this->Authentication->getIdentity();
					$observations->user_id = $user->id;
					$observations->observation = sprintf('Se agregó una nueva constancia. (%s)', $affiliateCertificate->affiliate->full_name);
					$this->Observations->save($observations);

					$this->Flash->success(__('La constancia de afiliación ha sido guardada.'));

					return $this->redirect(['action' => 'index']);
				}
				$this->Flash->error(__('La constancia de afiliación no pudo ser guardada. Por favor, intente nuevamente.'));
			}else{
				$this->Flash->error(__('Debe seleccionar un archivo de imagen.'));
			}
        }
        $enterprises = $this->AffiliateCertificates->Enterprises->find('list', ['limit' => 200]);
		$this->set(compact('affiliateCertificate', 'enterprises'));
		$this->set('movement', ['anulada' => 'anulada', 'recibida' => 'recibida', 'archivada' => 'archivada']);
    }

And this is the code of the template:

<?php
/**
 * @var \App\View\AppView $this
 * @var \App\Model\Entity\AffiliateCertificate $affiliateCertificate
 */
?>
<div class="row">
    <aside class="column">
        <div class="side-nav">
            <h4 class="heading"><?= __('Acciones') ?></h4>
            <?= $this->Html->link(__('Listar Constancias de Afiliación'), ['action' => 'index'], ['class' => 'side-nav-item']) ?>
        </div>
    </aside>
    <div class="column-responsive column-80">
        <div class="affiliateCertificates form content">
            <?= $this->Form->create($affiliateCertificate, ['type' => 'file']) ?>
            <fieldset>
                <legend><?= __('Agregar Constancia de Afiliación') ?></legend>
                <?php
                    echo $this->Form->control('enterprise_id', ['label' => 'Empresa', 'empty' => '- Seleccione una empresa -', 'options' => $enterprises, 'id' => 'enterpriseId']);
                    echo '<div class="inputOculto" style="display:none">'.$this->Form->control('affiliate_id', ['label' => 'Afiliado', 'id' => 'affiliateId']).'</div>';
                    echo $this->Form->control('start_date', ['label' => 'Fecha de inicio']);
                    echo $this->Form->control('reception_date', ['label' => 'Fecha de recepción']);
                    echo $this->Form->control('discount_request_url', ['label' => 'Solicitud de descuento', 'type' => 'file']);
                    echo $this->Form->control('movement_constancy', ['label' => 'Movimiento de constancia', 'options' => $movement]);
                    echo $this->Form->control('observations', ['label' => 'Observaciones']);
                ?>
            </fieldset>
            <?= $this->Form->button(__('Submit')) ?>
            <?= $this->Form->end() ?>
        </div>
    </div>
</div>

<script>
$(function(){
	$('#enterpriseId').change(function(){
		$.ajax({
			method:"POST", 
			url:"<?= $this->Url->build(['controller' => 'AffiliateCertificates', 'action' => 'add']) ?>",
			data:{
				enterprise_id:$(this).val()
			},
			success: function(data) {
				$('#affiliateId').html(data);
				$('.inputOculto').show();
			},
			headers:{
				'X-CSRF-Token':$('meta[name="csrfToken"]').attr('content')
			}
		})
	})
})
</script>
2

return $this->response->withStringBody('Your string goes here');

3

Thank you for the answer, now it functions perfect