Edit File: class-smtp.php
<?php /** * The SMTP class has been moved to the wp-includes/PHPMailer subdirectory and now uses the PHPMailer\PHPMailer namespace. */ _deprecated_file( basename( __FILE__ ), '5.5.0', WPINC . '/PHPMailer/SMTP.php', __( 'The SMTP class has been moved to the wp-includes/PHPMailer subdirectory and now uses the PHPMailer\PHPMailer namespace.' ) ); require_once __DIR__ . '/PHPMailer/SMTP.php'; class_alias( PHPMailer\PHPMailer\SMTP::class, 'SMTP' );