How to fix Hyper-V cannot be installed a hypervisor is already running

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

The above command installs only the hypervisor but does not install the administration tools, if you want 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

you want to create a cluster then you need to install the Cluster service, run: 
Install-WindowsFeature RSAT-Clustering -IncludeAllSubFeature 

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


Restart to finalize the virtual machine with the command:
Restart-Computer