Currently if you want to build for Universal Windows 10 Applications (a.k.a UWP), you have to do it on Windows 10.
Unity supports three UWP: X86, X64 and ARM.
In order to build applications for Universal Windows 10 application, Visual Studio 2015 RTM installation is required, the minimum version is 14.0.23107.0
Is something described here not working as you expect it to? It might be a Known Issue. Please check with the Issue Tracker at issuetracker.unity3d.com.