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In Woocommerce, I used Add a new custom checkout field before billing details in Woocommerce? from @LoicTheAztec that works perfectly.

I'm having trouble displaying the information in the admin order page.

Then I tried using Add order metadata to WooCommerce admin order overview, making some changes in the code to match with my needs:

// display the extra data in the order admin panel
function kia_display_order_data_in_admin( $order ) ?>
 <div class="order_data_column">
 <h4><?php _e( 'Extra Details' ); ?></h4>
 <?php 
 echo '<p><strong>' . __( 'Some field' ) . ':</strong>' . get_post_meta( $order->id, '_my_field_name', true ) . '</p>';?>
 </div>
<?php 
add_action( 'woocommerce_admin_order_data_after_order_details', 'kia_display_order_data_in_admin' );?>

Extra Details and Some field: appears, but the custom field value isn't displayed.

I'm still trying to figure out what I did wrong here. Any suggestions on where I should look?

Update 3: Try the following lightly revisited code:

add_action( 'woocommerce_checkout_before_customer_details', 'custom_checkout_fields_before_billing_details', 20 );
function custom_checkout_fields_before_billing_details()
 $domain = 'woocommerce';
 $checkout = WC()->checkout;

 echo '<div id="my_custom_checkout_field">';

 echo '<h3>' . __('My New Fields Section') . '</h3>';

 woocommerce_form_field( '_my_field_name', array(
 'type' => 'text',
 'label' => __('My 1st new field', $domain ),
 'placeholder' => __('Please fill in "my 1st new field"', $domain ),
 'class' => array('my-field-class form-row-wide'),
 'required' => true, // or false
 ), $checkout->get_value( '_my_field_name' ));

 echo '</div>';


// Custom checkout fields validation
add_action( 'woocommerce_checkout_process', 'custom_checkout_field_process' );
function custom_checkout_field_process() 
 if ( isset($_POST['_my_field_name']) && empty($_POST['_my_field_name']) )
 wc_add_notice( __( 'Please fill in "My 1st new field".' ), 'error' );


// Save custom checkout fields the data to the order
add_action( 'woocommerce_checkout_create_order', 'custom_checkout_field_update_meta', 10, 2 );
function custom_checkout_field_update_meta( $order, $data )
 if( isset($_POST['_my_field_name']) && ! empty($_POST['_my_field_name']) )
 $order->update_meta_data( '_my_field_name', sanitize_text_field( $_POST['_my_field_name'] ) );


// Display the extra data in the order admin panel
add_action( 'woocommerce_admin_order_data_after_order_details', 'display_order_data_in_admin', 10, 1 );
function display_order_data_in_admin( $order )
 if( $value = $order->get_meta( '_my_field_name' ) ) 
 echo '<div class="order_data_column">
 <h4>' . __( "Extra Details", "woocommerce" ) . '</h4>
 <p><strong>' . __( 'Some field', "woocommerce" ) . ':</strong> ' . $value . '</p>
 </div>';
 

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