Process the checkout and set error message

functions.php

<?php
//* Do NOT include the opening php tag shown above. Copy the code shown below.

//* Process the checkout
 add_action('woocommerce_checkout_process', 'wps_select_checkout_field_process');
 function wps_select_checkout_field_process() {
    global $woocommerce;

    // Check if set, if its not set add an error.
    if ($_POST['daypart'] == "blank")
     wc_add_notice( '<strong>Please select a day part under Delivery options</strong>', 'error' );

 }