You have to insert Windows Server 2012 DVD into DVD Reader. After you can install .Net FrameWork 3.5.
Installation Steps
- insert Windows Server 2012 DVD or iso to D Drive.
- Open Add Roles and Features Wizard From Server Manager.
- Select .NET Framework 3.5 Features.
- Select Specify an alternate source path and select path that is D:\Sources\SxS.
- Finally click Install button.