messageManager

use \Magento\Framework\Message\ManagerInterface;

use below code in your file to add messages,

protected _messageManager;

public function __construct(
    \Magento\Framework\View\Element\Template\Context $context, 
    \Magento\Framework\Message\ManagerInterface $messageManager
) {
    parent::__construct($context);
    $this->_messageManager = $messageManager;
}

and then add below functions in your methods to show messages:

$this->_messageManager->addError(__("Error Message"));
$this->_messageManager->addWarning(__("Warning"));
$this->_messageManager->addNotice(__("Notice"));
$this->_messageManager->addSuccess(__("Success Message"));

I hope this will help you fixing your issue.