Offline VCF environments/dark sites.
VCF on VxRail 5.x
NSX-T bundle will not be available for use until the upgrade plan has a VxRail build number.
Upload the appropriate VxRail bundle to SDDC manager with the Bundle Transfer Utility and refresh the SDDC manager UI.
These knowledge base article will assist with resolution and further troubleshooting:
Troubleshooting: Resolve VxRail Upgrade Failure when upgrading from VCF on VxRail 5.X to 5.Y
https://knowledge.broadcom.com/external/article?articleNumber=323978
SDDC upgrade wizard fails to list NSX-T
https://knowledge.broadcom.com/external/article/377484/sddc-upgrade-wizard-fails-to-list-nsxt.html