.NET Framework 3.5 is not in the cached installers in Windows Server 2012 R2. When installing .NET Framework 3.5 you need to specify the \Sources\SXS folder on the DVD.
You can use the Server Manager wizard, DISM or Add-WindowsFeature commands to install .NET Framework 3.5. But this often fails after installing a bunch of updates. It says because it can’t find the sources.
Here’s how to fix it:
|1. Stop Windows Update Service|
|2. Disable Windows Update Service|
|3. Remove update KB2966828|
|4. Open CMD with admin rights|
5. dism.exe /online /enablefeature /all /featurename:NetFX3 /source:”D:\Sources\SXS” /LimitAccess
Where D:\ is de drive letter of the mouted DVD
There are multiple versions of the Windows 2012 R2 DVD available so make sure to use the one that you installed Windows Server 2012 R2 with.