Upgrading from 1.9 to 2.0 error

I am upgrading from 1.9 to 2.0 and have 1 error that is still limiting the upgrade. Any help on this issue would be helpful.

Build FAILED.

       "C:\Inetpub\wwwroot\build\nop.proj" (Deploy target) (1) ->
       "C:\Inetpub\wwwroot\NopCommerce.sln" (default target) (2) ->
       "C:\Inetpub\wwwroot\Libraries\Nop.Services\Nop.Services.csproj" (default
        target) (5) ->
       (GenerateSerializationAssemblies target) ->
         SGEN : error : An attempt was made to load an assembly with an incorre
       ct format: C:\Program Files\Reference Assemblies\Microsoft\Framework\.NE
       TFramework\v4.0\Microsoft.CSharp.dll. [C:\Inetpub\wwwroot\Libraries\Nop.
       Services\Nop.Services.csproj]