No .metalmap found for SF_METAL

Does anyone know what could cause the error:

LogMetal: Display: No .metalmap file found for SF_METAL!

This output is displayed in the function:

FRHIShaderLibraryRef FMetalDynamicRHI::RHICreateShaderLibrary(EShaderPlatform Platform, FString FolderPath)

I have tried it in the unofficial UE4-19 and its the same thing and it always occurs when trying to actually run the deployed package on the iPhone.

Please let me know if you need any further info - I will continue to debug this but it is very deep into the shader world…

Also just so that you know, the Editor version works with UE4-17 but fails with the above when I deploy to iPhone and try and run it.I am using XCode 9.2 as the debugger and ProjectLauncher as the build / deploy / run

Thanks in advance if anyone can help here