Why woocommerce order not complete after payment?

Woocommerce said "If the payment has been successful and the order does not contain Digital or Downloadable products, the order will be set to Processing" http://prntscr.com/md48nn

So you need disable manager stock in your site WP admin page => Woocoomerce => Settings => Products => Inventory http://prntscr.com/md48vr

Please see more info https://docs.woocommerce.com/document/managing-orders/