If this is the plugin you’re using, I recommend asking at https://wordpress.org/support/plugin/royal-mail-woocommerce-shipping-calculator/#new-post so the plugin’s / theme’s developers and support community can help you with this.

On the other hand, if you’re using https://woocommerce.com/products/royal-mail/, then support of all commercial themes and plugins is out of scope for these forums. While we will not close your post, we also cannot guarantee any support at all, be that by volunteers or the developers. We recommend you instead use the developer’s official contact for their premium offerings, as that is the most direct line..

https://woocommerce.com/my-account/create-a-ticket/