Occationally I come across Windows installations that refuse to activate the .net 3.5 feature.
There could be various reasons preventing this, one could be WSUS.
A simple workaround is to download the ISO for the running Windows version (winver)
Mount this and run the following from an elevated cmd
Dism /online /enable-feature /featurename:NetFx3 /All /Source:e:\sources\sxs /LimitAccess
Should sort this issue