The notification message is regarding the active support contract on the ZD. If you do not have active support on the ZD, you would not be able to upgrade the ZD.
Download the entitlement file from the support site using the below link and upload that .spt file under Administer> Support on the ZD. Then try to upgrade.
If you do not have active support contract, you will get a 90 day warranty entitlement file.