AWS allows the creation of Sub-Account CUR files using AWS Conductor. Unfortunately Sub-Account CURs are not supported and can't be used to configure a Partner Customer billing block
Customers must be configured using standard CUR files as outlined here:
https://techdocs.broadcom.com/us/en/vmware-tanzu/cloudhealth/tanzu-cloudhealth/saas/tnz-cloudhealth/working-with-partner-platform-of-tanzu-cloudhealth-aws-managed-service-providers.html