Below are the few issues/error message I have come across in setting up fault Tolerance (FT)
Problem : Cannot add a host with virtual machines that have Fault Tolerance turned On to a non-HA enabled cluster.
Solution: Verify HA is turned on in cluster level settings.
Problem : In the Alarms you might see sometimes Host connection and Power state.. (Sometimes this may direct you in different directions)
Solution : This technically means you need to make sure managed IP address in the administration of Vsphere section is updated properly with the vCenter server IP address.