The error is saying that you have 2 DLLs called P31RestKit in your project, one at "Assets/Plugins/P31RestKit.dll" and one at "Assets/Plugins/WSA/P31RestKit.dll".
The process for dealing with plugins for Windows Unviersal has changed in Unity 5 from Unity 4. (It actually seems to change with every update and patch fix!). In Unity 5 you have to specify which platforms each dll will be used on.
To do this select each dll in the Project window and change their settings. For the dll in the standard plugins folder just deselect the WSAPlayer. For the dll in the WSA folder set the platform just to WSAPlayer. In additon also tick the box that says 'Don't process" and in the Placeholder drop down, navigate to "Assets/Plugins/P31RestKit.dll".
That should sort you out. It is worth pointing out that Prime31 is (maybe already has) dropped support for the Windows Universal apps platform. This is partly due to various issues with the Unity/Windows Universal/Plugins combination, which as I said is a bit of a nightmare at the moment.
For reference the plugin vendor's documentation on this is here The advice is slightly out of date though and for Unity 4 I believe.