Process custom field in checkout

functions.php

<?php
/**
 * Process the checkout
 */
add_action('woocommerce_checkout_process', 'some_custom_checkout_field_process');
 
function some_custom_checkout_field_process() {
    // Check if the field is set, if not then show an error message.
    if ( ! $_POST['some_field_name'] )
        wc_add_notice( __( 'Please enter something here.' ), 'error' );
}
?>