This issue may occurs when vCenter is unable to install FDM agent in ESXi host due to any reason. If FDM agent is not exist, vCenter will generate below error:

Error:-

Cluster will show error as "Cannot find vSphere HA master agent"

vSphere HA State in VC Console Summary Page will show error "vSphere  HA State: Initialization Error"

vCenter will generate error while reconfiguring HA. "Cannot install the vCenter agent service. Unknown installer error"

Refer to below logs to analyze this issue.

/var/log/vpxa.log
/var/log/fdm.log
/var/run/log/fdm-installer.log

Solution:-

Fix this issue by reinstalling FDM Agent.

- Remove existing agent by using below command.

esxcli software vib remove -n vmware-fdm

- Install new FDM agent by Reconfiguring HA on cluster.

- If Reconfiguring HA doesn't work, Remove Hosts from Cluster, and re-add it back. Enable HA on Cluster.