Legacy FT VMs can exist only on ESXi hosts that are running on vSphere versions earlier than 6.5.
ESXi hosts prior to version 6.5 supported vSphere Fault Tolerance based on a different technology. If you are using this form of Fault Tolerance and need to continue doing so, we recommend you reserve a vCenter 6.0 instance to manage the pool of pre-6.5 hosts required to run these VMs. vCenter 6.0 was the last version fully capable of managing legacy FT protected VMs. For more information on Legacy Fault Tolerance, see vSphere Availability 6.0 documentation.