functions.php

<?php if ( ! function_exists( 'yith_pos_customization_order_status_to_complete' ) ) {
    add_filter( 'woocommerce_payment_complete_order_status', 'yith_pos_customization_order_status_to_complete', 10, 3 );

    function yith_pos_customization_order_status_to_complete( $status, $order_id, $order ) {
        if ( absint( $order->get_meta( '_yith_pos_order' ) ) ) {
            $status = 'completed';
        }

        return $status;
    }
}