1. From Desktop, in the search window enter the text Local Group Policy Editor or Group Policy Management.
2. Once it opens, expand Computer Configuration tree on the left hand, expand Administrative Templates, and then select System.
3. Do Not Expand System, just click select.
4. On the righthand pane, scroll down to find the text “Specify settings for optional component installation and component repair Group Policy” setting, and then select Enabled radio button on the top left corner.
5. In the bottom group, Select the Contact Windows Update directly to download repair content instead of Windows Server Update Services (WSUS) radio button.
6. Click Apply and OK and try to install .net framework 3.5 from Server Roles. It takes about a minute or two, but, eventually, the system will install .Net Framework 3.5.
Make sure Windows Updates Service is set to Manual or Automatic to apply this fix.