I was trying to create Hyper-V lab top of VMware Platform and got an error Hyper-V cannot be installed:The hypervisor is already running.

First thing you need to do is to open PowerShell and then run the following command to install the Hyper-V :

 Enable-WindowsOptionalFeature –Online -FeatureName Microsoft-Hyper-V –All -NoRestart 

To manage the Hyper-V within the virtual machine you must install RSAT for Hyper-V to do this run the following command:

 Install-WindowsFeature RSAT-Hyper-V-Tools -IncludeAllSubFeature

To create a cluster then you need to install the Cluster service,  run: 

 Install-WindowsFeature RSAT-Clustering -IncludeAllSubFeature 

Optionally you can install Multipath I/O to enable multiple paths to the storage, if this is your case follow:

Install-WindowsFeature Multipath-IO

Now restart the server and you are Done ! Now just open the console of the Hyper-V Manager and create a virtual machine.