Outbound 403: Zendesk Task to GC Contact List Mapping

Could someone clarify why the Outbound Campaign API returns a 403 Forbidden error when importing Zendesk task contacts into Genesys Cloud? The Zendesk tasks map perfectly to GC contact attributes, but the migration script fails at the final sync step.

POST /api/v2/outbound/contactlists/{contactListId}/contacts returned 403: Access Denied