This article provides information on why vSphere HA configuration fails to install or configure vSphere HA agent on the ESXi after rolling back the ESXi from 6.7 to the previous version
Symptoms:
VMware vCenter Server Appliance 6.7.x
VMware vSphere ESXi 6.7
VMware vCenter Server 6.7.x
VMware vSphere ESXi 6.0
VMware vSphere ESXi 6.5
After the ESXi rollback from version 6.7, the new tools-light VIB does not revert to the earlier version. As a result, the VIB becomes incompatible with the rolled back ESXi host causing these issues. Ref (https://techdocs.broadcom.com/content/dam/broadcom/techdocs/us/en/pdf/vmware/vsphere/vsphere/vSphere-6-7-Release-Notes/vsphere-esxi-vcenter-server-67-release-notes.pdf)
Because of the above behavior the "vmware-fdm" vib on the ESXi fails removal, preventing the installation of the latest FDM vib from vCenter causing the vSphere HA to fail with error "vSphere HA agent cannot be installed or configured"
To resolve the issue