Apparently, there’s a bug in Windows Server 2012 when trying to add the .NET Framework 3.5 from Server Manager. In order to get it to work, I had to enable the feature via the Command Prompt.

This is the command that I had to type:

dism /online /enable-feature /featurename:NetFX3 /all /Source:d:\sources\sxs /LimitAccess

Note-1: Source should be the Windows installation disc. In my case, this was located on D:
Note-2: CMD or powershell must be administrator rights.