How to trigger WooCommerce order complete email?

You can try this

$mailer = WC()->mailer();
$mails = $mailer->get_emails();
if ( ! empty( $mails ) ) {
    foreach ( $mails as $mail ) {
        if ( $mail->id == 'customer_completed_order' ) {
           $mail->trigger( $order->id );
        }
     }
}