As MSDN statement for MSBuild Error MSB3187:
This warning is generated when the application's target platform (processor architecture) is set to neutral (MSIL) and the referenced assembly is not neutral, or if the application's architecture is not neutral and the dependency is neutral. Also, if both are not platform-neutral, then their architecture must match. In addition, application architecture and entry point assembly architecture must always match.
Since your application is compatible with "Any CPU" but you should have a dependency on a project or .dll assembly that is either x86 or x64. Visual Studio will give this waring.
To make the warning go away, you should make sure that the application's target platform (processor architecture) matches all referenced assemblies and the entry point assembly architecture.
Hope this could help you