Like mentionned in the error message, you need to create an admin class for the ActionConfig entity with the function configureFormFields

Bellow an exemple of code.

<?php

namespace App\Admin;

use Sonata\AdminBundle\Admin\AbstractAdmin;
use Sonata\AdminBundle\Form\FormMapper;

final class ActionConfigAdmin extends AbstractAdmin
{
    protected function configureFormFields(FormMapper $form): void
    {
        $form->add('your_field_name');
    }
}

Don't forget to register it in the config/services.yaml with your name convention

# config/services.yaml
services:
    app.admin.action.config:
        class: App\Admin\ActionConfigAdmin
        tags:
            - {
                  model_class: App\Entity\ActionConfigAdmin,
                  name: sonata.admin,
                  manager_type: orm,
                  group: actrionAdmin,
                  label: actrionAdmin,
              }
        public: true

Documentation : https://docs.sonata-project.org/projects/SonataAdminBundle/en/4.x/reference/form_types/#sonata-adminbundle-form-type-admintype