If you are installing the full GUI on your Server core 2012 and receive this error, it is an easy fix.
Install-WindowsFeature : The request to add or remove features on the
specified server failed.
Installation of one or more roles, role services, or features failed.
The source files could not be downloaded.
This simply means that your installation files need to be made available to PowerShell to do this. To correct this, insert your installation media or map a drive to the installation source on your network. Now add he –Source parameter to your Install-WindowsFeature cmdlet pointing to Windows folder to complete the installation
Imstall-WindowsFeature –IncludeAllSubfeature User-Interfaces-Infr –Source <SourcePath>