Was trying to enable the .NET 3.5 Framework Feature on Windows Server 2012.

Received this error:
The source files could not be found
or
The request to add or remove features on the specified server failed. ……..

I tried the command line idea, PowerShell, and copying the files from the install media.
All failed.

The issue??
No internet connection!

If you hit all the wall I did, try connecting the box / VM to the web.