vSphere HA agent should normally install in vSphere through unconfiguring then reconfiguring vSphere High Availability (vSphere HA) in vCenter Server at the cluster level or at the ESXi host level.
Problems with this not working can often be solved by manually uninstalling, then reinstalling, the vSphere HA agent directly on an affected ESXi host. The VMware-fdm-uninstall.sh script may assist with cleanly removing the vSphere HA agent VIB and related configuration files.
FDM stands for Fault Domain Manager. This was the original name for the vSphere HA feature.
Error: "vSphere HA agent cannot be correctly installed or configured"