I struggled with it as well. Changing validation in functions.php didn’t work, but what helped is modification of wp_options table for wooccm_billing setting. Find billing_city part of the setting and replace s:8:"validate";a:1:{i:0;s:8:"postcode";} and try to replace with s:8:"validate";a:0:{}. Obviously it’s risky to modify DB by hand, but I found that’s the only option if I want to continue using the plugin.
