Run a function on custom button click in woocommerce admin order page

There are some missing things in your code and an error in your last function where count(couriers); need to be instead count($couriers);.

// Display an action button in admin order list headeradd_action( 'manage_posts_extra_tablenav', 'admin_order_list_top_bar_button', 20, 1 );function admin_order_list_top_bar_button( $which ) {    global $pagenow, $typenow;    if ( 'shop_order' === $typenow && 'edit.php' === $pagenow && 'top' === $which ) {        ?>        <div class="alignleft actions custom">            <button type="submit" name="import_courier" style="height:32px;" class="button" value="yes"><?php                echo __( 'Import Couriers', 'woocommerce' ); ?></button>        </div>        <?php    }}// Trigger an action (or run some code) when the button is pressedadd_action( 'restrict_manage_posts', 'display_admin_shop_order_language_filter' );function display_admin_shop_order_language_filter() {    global $pagenow, $typenow;    if ( 'shop_order' === $typenow && 'edit.php' === $pagenow &&    isset($_GET['import_courier']) && $_GET['import_courier'] === 'yes' ) {                ## -------- The code to be trigered -------- ##                update_shipping_couriers_meta_field();                ## -------------- End of code -------------- ##    }}// Your function that will be triggered on button pressfunction update_shipping_couriers_meta_field() {    $dir = __DIR__;    $couriers = file( $dir . '/import-couriers.csv', FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES );    $count = count($couriers);    $i = 1;        do {        if ( ! empty( $couriers ) ) {            foreach ( $couriers as $a ) {                if ( ! empty( $a ) ) {                    $rows = explode(';', $a);                        update_post_meta( intval($rows[0]), '_shipping_couriers', $rows[1] );                }                $i++;            }        }    }     while ( $i <= $count );}

Code goes in functions.php file of your active child theme (or active theme). Tested and works.

Based from: Add a button on top of admin orders list in woocommerce