Custom field in checkout

Can you share the code you’re using for it?

This is the code I am using:

/**
 * Add the field to the checkout
 */
add_action( 'woocommerce_after_order_notes', 'my_custom_checkout_field' );

function my_custom_checkout_field( $checkout ) {

    echo '<div id="personalisecookie"><h2>' . __('Personalise your cookie') . '</h2>';

    woocommerce_form_field( 'my_field_name', array(
        'type'          => 'text',
        'class'         => array('my-field-class form-row-wide'),
        'label'         => __('Our cookies can be personalised *up to 25 characters.'),
        'placeholder'   => __('Enter your details here'),
        ), $checkout->get_value( 'personalisation' ));

    echo '</div>';

}

/**
 * Update the order meta with field value
 */
add_action( 'woocommerce_checkout_update_order_meta', 'my_custom_checkout_field_update_order_meta' );

function my_custom_checkout_field_update_order_meta( $order_id ) {
    if ( ! empty( $_POST['personalisation'] ) ) {
        update_post_meta( $order_id, 'personalisation', sanitize_text_field( $_POST['personalisation'] ) );
    }
}

/**
 * Display field value on the order edit page
 */
add_action( 'woocommerce_admin_order_data_after_billing_address', 'my_custom_checkout_field_display_admin_order_meta', 10, 1 );

function my_custom_checkout_field_display_admin_order_meta($order){
    echo '<p><strong>'.__('personalisation').':</strong> ' . get_post_meta( $order->id, 'personalisation', true ) . '</p>';
}

[Moderator Note: Please post code between backticks or use the code button. Your posted code may now have been damaged by the forum’s parser.]

It looks like you have one error in your code, in the line below, you need to replace my_field_name with personalisation:

woocommerce_form_field( 'my_field_name', array(

Everything else should work from there.