If the unresolved symbol is a Windows Runtime free function, such as RoInitialize, then you'll need to explicitly link the WindowsApp.lib umbrella library in your project. The C++/WinRT projection depends on some of these free (non-member) functions and entry points. If you use one of the C++/WinRT Visual Studio Extension (VSIX) project templates for your application, then WindowsApp.lib is linked for you automatically. Otherwise, you can use project link settings to include it, or do it in source code.
#pragma comment(lib, "windowsapp")
It's important that you resolve any linker errors that you can by linking WindowsApp.lib instead of an alternative static-link library, otherwise your application won't pass the Windows App Certification Kit tests used by Visual Studio and by the Microsoft Store to validate submissions (meaning that it consequently won't be possible for your application to be successfully ingested into the Microsoft Store).
Relatively rare: if the unresolved symbol is an API from the Windows namespace headers for the C++/WinRT projection (in the winrt namespace), then the API is forward-declared in a header that you've included, but its definition is in a header that you haven't yet included. Include the header named for the API's namespace, and rebuild. For more info, see C++/WinRT projection headers.