If you try to turn on vSphere Fault Tolerance for a powered-on VM, this operation can fail.
When you select Turn On Fault Tolerance for a powered-on VM, the operation fails and you see an Unknown error message.
This operation can fail if the host that the VM is running on has insufficient memory resources to provide fault tolerant protection. vSphere Fault Tolerance automatically tries to allocate a full memory reservation on the host for the VM. Overhead memory is required for fault tolerant VMs and can sometimes expand to 1 to 2 GB. If the powered-on VM is running on a host that has insufficient memory resources to accommodate the full reservation plus the overhead memory, trying to turn on Fault Tolerance fails. Subsequently, the Unknown error message is returned.
Choose from these solutions:
Free up memory resources on the host to accommodate the VM's memory reservation and the added overhead.
Move the VM to a host with ample free memory resources and try again.