Symptoms
There is a Reseller with "Customer Management" Office 365 subscription, related to Syndication scenario application instance. But he desires to manage CSP customers also, and there is another - CSP application instance created.
On attempt to subscribe a Reseller for a second "Customer Management" subscription according to the How to Upgrade Office 365 Subscriptions through Provider/Reseller Control Panel documentation article, the following error appears on subscription creation wizard screen:
"code": 500,
"message": "Tenants should be specified for account with Office 365\r\nParameter name: tenants",
"additionalNonlocalizableMessage": ""
Cause
There is a known limitation: an account (Customer or Reseller) cannot own several subscriptions, related to different application instances of the same APS application.
Software-related issue #APSA-17566: "Office365: On attempt to add "Customer Management" subscription to a Reseller error appears" has been created to change current package behavior.
Resolution
Contact TAM or pta@odin.com to trace the status of the issue.