This error usually occurs when you try to install ".Net Framework 3.5" on a Windows 8/Windows Server 2012 and don't have internet connection established.
Or your internet connection's being blocked by Firewall, ISP, or Administrator. First of all, check if your Firewall isn't blocking such services as "Windows Update".
info on how to fix this. There are several ways of fixing such problems, in the "Resolution" section. It is basically a "step-by-step" guide.