I got message “Certificate management temporarily not available” when testing on resellerclub demo OrderBox
Once added a certificate order on the OrderBox demo plattform http://cp.onlyfordemo.com/reseller, log into your WHMCS admin area and select the certificate order under the "Products/Services" Tab.
Add at the end of the domain-name in the field "Domain" and in the custom field for the common-name, the suffix ".onlyfordemo.com"
Now you can manage the certificate and use any admin functions including the terminate function.
Please note: Once terminated the order you need to rename "secure.mydomain.com.onlyfordemo.com" again back to "secure.mydomain.com" to re-create the order.