Hi. That's the error you'd normally see if you somehow ended up starting a store on either the wrong platform or on an unsupported platform. Windows Phone is a supported platform for Unity IAP, so seeing this error suggests that something went wrong in the build. The usual problems here are if the IAP plugin directories were re-arranged or the .meta files for the store DLLs were modified or deleted. There are two different files named `winrt.dll` in the plugin tree and in this case you're hitting the wrong one. For future IAP issues you may also want to check the Unity IAP Forum. The IAP development team handles both the general and platform-specific aspects of Unity IAP and monitors that forum closely. Finally, IAP on an emulator isn't really a well-supported configuration and could cause unexpected behavior on some platforms. That's not the problem you're seeing here, but something to consider while testing.
@GLeBaTi UWP/IL2CPP will support Unity IAP in the 5.6 release of Unity. Note that you need to use the .NET 4.6 API compatibility level to make this work though.