VMware Cloud Foundation 5.1.0.0
The issue is encountered due to a feature - feature.vcf.esxi.smartnic.enable. The feature includes a check for at least 3 hosts, if there are lesser than 3 hosts, the issue will be observed.
This is an issue found on SDDC 5.1.0.0 with a vLCM-based domain only.
Upgrade the SDDC Manager to 5.1.1.0 or later
Workaround -
1. Commission 3 hosts to the SDDC and then create a cluster. Commission Hosts
2. If there are 3 hosts, all hosts will be visible in the wizard, and then select 2 hosts that are required to be added to the cluster. (This additional host can be later decommissioned if not required.) Add a Host to a vSphere Cluster Using the SDDC Manager UI