Simply create a mu-plugin folder in your wp-content folder and add this woo-autocomplete.php file.

woo-autocomplete.php

<?php
defined( 'ABSPATH' ) or die( 'Go Away!' );
/**
 * Plugin Name: WooCommerce AutoComplete Plugin
 * Description: AutoComplete all sales on WooCommerce.
 * Author:      Will Preston
 * License:     GNU General Public License v3 or later
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
 */

/* Auto Complete All WooCommerce Orders */
add_action( 'woocommerce_thankyou', 'woocommerce_auto_complete_order' );

function woocommerce_auto_complete_order( $order_id ) { 
  if ( ! $order_id ) {
    return;
  }
 
  $order = wc_get_order( $order_id );
  $order->update_status( 'completed' );
}