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https://mage2-blog.com/magento-2-display-success-and-error-messages/

protected messageManager;
 
public function __construct(
    ... 
    \Magento\Framework\Message\ManagerInterface $messageManager,
    ...
){
    $this->messageManager = $messageManager;
    ...
}

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