In the Site Settings page, use SDDC manager as the backup server to backup NSX and SDDC manager itself.
When running "Backup Now" on SDDC manager, it fails to start the job with such error.
VCF9.0 GA
It's a known issue in VCF9.0GA, it will be fixed in the future.
Follow the KB to use API to configure backup on SDDC manager
https://knowledge.broadcom.com/external/article?articleNumber=389896