Could not load System.Core 4.0 in Windows 10 Universal App

I am having an issue with the latest version of Window 10 Universal App and PCL libraries

This DLL must be on the system because a previous version of a UWP10 worked with the same library. I have tried to download .NET Framework 4 KB fixes but they won't be installed on Windows 10 stating :

• Another Version of this product already exist

• OR Microsoft .NET Framework 4 is already a part of this operating system. Same or higher version of .NET Framework 4 has already been installed on this computer.

  {"Could not load file or assembly 'System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)":"System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"}

I've ever had same problem with you.

When you make an app with selecting Blank app (Universal Windows), the app has reference System.Core like below image.

But, when you make an app with selecting Blank app (Xamarin.Forms Portable), there is no System.Core.

As a result, you could not use the .dll you use with uwp10. But if you want to use the .dll, you need to port through PCL API to get rid of using System.Core API.