I have purchased an add-on from Plesk Online Store. The main Plesk license was purchased from a reseller or partner. When I try to download the add-on key from my Online Store account, I get the error below:
Sorry, the service module failed to return valid data. Please try again later.
When an add-on is purchased for a Plesk license obtained from third party, a Feature Key is created. Unlike regular license, this one cannot be downloaded through the Online Store account, because Feature Key does not cover regular licensing options. Instead, it only marks that a third party Plesk license now has an additional feature. A Feature Key cannot be used separately from its parent Plesk key.
In order to use the purchased add-on, you need to either install the parent Plesk license again, or retrieve it through Plesk interface in case if this key has already been installed. Once key is installed or retrieved, Plesk discovers the new option that has been added as a Feature Key.