Cannot enable the Microsoft .NET Framework 3.5 feature on Windows Server 2012 R2

Install-WindowsFeature –name NET-Framework-Core –source F:\sources\sxs
F:\ is the mounted Windows Server 2012 R2 ISO. 

Find VM's GUID

• Open PowerShell As Admin.

Get-WmiObject -Namespace root\virtualization\v2 -class msvm_computersystem | select elementname, processid, name| ft -auto

Result:
elementname     processid name
-----------     --------- ----
***DMN***                 HOST Machine
VMNAME1         3696      1614986B-5D42-4A5F-A4B6-04CD60F88802
VMNAME2         5376      485B2AE7-CA18-4C20-A2FE-E403F4EAF5CE

then your can take related actions: 

Kill related vmwp.exe <GUID>.
taskkill /PID 5376 /F


Or From Task Manager: