Sometimes when you add a host to a VMware vCenter HA Cluster the operation fails with a timeout for cluster election. That is still happening when running VMware vCenter 6.0 and adding 5.5 hosts to it.
The normal way is to start a Reconfigure for vSphere HA on the troubled ESXi Host within the cluster.
If you experience that the error message continues to pop up and the ESXi host is not getting a green status within the cluster you can always try to re-add the host. Unfortunately most of the time that doesn´t solve the issue and getting the host out of a cluster is always painful (maintenance mode).
The quick and dirty way solving that issue is to disable the HA cluster and enable it again!
In most cases we experienced so far, this helps resolving the issue real quick and painless.