Magento2 Plugin – How to Overwrite Function Using Plugin Method

It happens when you have problem in your constructor.

Remove \Magento\Store\Model\ScopeInterface $scopeInterface from your constructor and use \Magento\Framework\App\Config\ScopeConfigInterface $scopeInterface instead of it.

Use the following to read your config:

$storeScope = \Magento\Store\Model\ScopeInterface::SCOPE_STORE;
return $this->_storeconfig->getValue('my/data/path', $storeScope);

Still not sure whether I can use plugin or not as per my functionality requirement. So as a workaround, I'm using preference. Would love to hear from someone the plugin_way of doing this. Preference way is as follow. Hope this helps someone.

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">    
    <preference for="Magento\User\Controller\Adminhtml\User\Validate" type="[vendor]\[module]\Controller\Rewrite\Adminhtml\User\Validate" />
</config>

And the rewritten controller is as follow.

namespace [vendor]\[module]\Controller\Rewrite\Adminhtml\User;

class Validate extends \Magento\User\Controller\Adminhtml\User\Validate
{
    /**
     * AJAX customer validation action
     *
     * @return void
     */
    public function execute()
    {
        $response = new \Magento\Framework\DataObject();
        $response->setError(0);
        $errors = null;
        $userId = (int)$this->getRequest()->getParam('user_id');
        $data = $this->getRequest()->getPostValue();

        if([some_validation_criteria_fails]) {
            $err = "You error message";
            $errors[] = $err; 
        }else{

            try {
                /** @var $model \Magento\User\Model\User */
                $model = $this->_userFactory->create()->load($userId);
                $model->setData($this->_getAdminUserData($data));
                $errors = $model->validate();
            } catch (\Magento\Framework\Validator\Exception $exception) {
                /* @var $error Error */
                foreach ($exception->getMessages(\Magento\Framework\Message\MessageInterface::TYPE_ERROR) as $error) {
                    $errors[] = $error->getText();
                }
            }

        }

        if ($errors !== true && !empty($errors)) {
            foreach ($errors as $error) {
                $this->messageManager->addError($error);
            }
            $response->setError(1);
            $this->_view->getLayout()->initMessages();
            $response->setHtmlMessage($this->_view->getLayout()->getMessagesBlock()->getGroupedHtml());
        }

        $this->getResponse()->representJson($response->toJson());
    }

}
?>

This will return error message as per your validation scenario.