11-custom-admin-notices.php

<?php
add_action( 'admin_notices', 'admin_notice__success' );
function admin_notice__success() {
	$class   = 'notice notice-success is-dismissible';
	$message = __( 'Completelly with success!', 'text_domain' );

	printf( '<div class="%1$s"><p>%2$s</p></div>', esc_attr( $class ), esc_html( $message ) );
}

add_action( 'admin_notices', 'admin_notice__error' );
function admin_notice__error() {
	$class   = 'notice notice-error';
	$message = __( 'An error has occurred.', 'text_domain' );

	printf( '<div class="%1$s"><p>%2$s</p></div>', esc_attr( $class ), esc_html( $message ) );
}