// for success
<div class="notice notice-success is-dismissible">
<p><?php _e( 'Done!', 'sample-text-domain' ); ?></p>
</div>
// for error
<div class="notice notice-error is-dismissible ">
<p><?php _e( 'Error message here!', 'sample-text-domain' ); ?></p>
</div>
function sample_admin_notice__success() {
?>
<div class="notice notice-success is-dismissible">
<p><?php _e( 'Done!', 'sample-text-domain' ); ?></p>
</div>
<?php
}
add_action( 'admin_notices', 'sample_admin_notice__success' );
# More advance version
function sample_admin_notice__error() {
$class = 'notice notice-error';
$message = __( 'Irks! An error has occurred.', 'sample-text-domain' );
printf( '<div class="%1$s"><p>%2$s</p></div>', esc_attr( $class ), esc_html( $message ) );
}
add_action( 'admin_notices', 'sample_admin_notice__error' );