This issue occurs when a previous import attempt failed against Cloud Director but the respective tables in the database have not been cleaned.
This issue has been resolved in VMware Cloud Director 10.6.1.1. See VMware Cloud Director 10.6.1.1 Release Notes
For a workaround, contact Broadcom Technical Support and note this Knowledge Article ID (385446) in the problem description.
For more information, see How to Submit a Support Request