Error:

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.

Use the “source” option to specify the location of the files that are required to restore the feature. For more information on specifying a source location,see http://go.microsoft.com/fwlink/?LinkId=243077. Error: 0x800f0906

Coretech Blog » Blog Archive » Why I can’t convert my Windows Server 2012 R2 Core to GUI
Here are the commands to update the source files with PowerShell

# Create folders
New-Item -Path C:\WS2012R2 -ItemType Directory
New-Item -Path C:\TEMP -ItemType Directory

# Copy the install.wim to temporary folder
Copy-Item H:\sources\install.wim C:\TEMP

# Remove the read-only attribute from the copied file
Set-ItemProperty -Path C:\TEMP\install.wim -Name IsReadOnly -Value $False

# Get the Windows Image Index
Get-WindowsImage -ImagePath C:\TEMP\install.wim

# Mount the Image
Mount-WindowsImage -Path C:\WS2012R2 -ImagePath C:\TEMP\install.wim -Index 2

# Add the Updates to Windows Image
$Updates = Get-ChildItem C:\Users\kaj\Desktop\Updates
foreach($item in $Updates){
Add-WindowsPackage -PackagePath $item.FullName -Path “C:\WS2012R2”
}

# Commit the changes
Dismount-WindowsImage -Path C:\WS2012R2 -Save -Verbose

source: http://blog.coretech.dk/kaj/why-i-cant-convert-my-windows-server-2012-r2-core-to-gui/