This issue is seen when deploying VCF 9 using the VCF installer and selecting an existing vCenter as the management domain vCenter.
Where the existing ESXI HOST are deployed with short name and not FQDN.
The deployment validation fails as Hostnames not recognized as FQDN by VCF Installer.
host HOST_SHORT_NAME: Host name is not a FQDN
Remediation: Update the Host name with a FQDN
VCF 9.0.0
ESX HOST was deployed with Short name instead of FQDN
Rename ESXI Host With FQDN