The reason could be that no billing address (billing_address) was assigned to your main organization.
In order to enable the LIVE mode for a managed_organization, a billing address for the main organization should be created via the Management API endpoint createBillingAddress. The main organization can then be updated providing the billing_address_id via the updateOrganization endpoint.
This process requires a user authentication via the Management API endpoint authenticate:
As soon as this has been done, the LIVE mode of the relevant organization can be enabled.
If you have further question with this regard, please contact us and provide all the important details (such as the ID of the managed_organization / main organization, error messages received, etc.).