error LNK2019: unresolved external symbol "void __cdecl GenerateConvenientWindowedResolutions

So I was modifying on UkismetSystemLibrary, to work just with c++ programming, but I stuck on that problem of linkage that i cant solve. Looking on google I cant find nothing related with this. I tried Linking the GameEngine.cpp Path where GenerateConvenientWindowdResolutions() is implemented on MyProject.buil.cs but no success.
On UkismetSystemLibrary this funcition is caled as extern on function GetConvenientWindowedResolution() and doesnt have any declaretion on GameEngine.h.

on “UkismetsystemLibray.cpp”

bool UAGSystem::GetConvenientWindowedResolutions(TArray<FIntPoint>& Resolutions)
	FDisplayMetrics DisplayMetrics;
	if (FSlateApplication::IsInitialized())
// Declared here as extern//
	extern void GenerateConvenientWindowedResolutions(const struct FDisplayMetrics& InDisplayMetrics, TArray<FIntPoint>& OutResolutions);
	GenerateConvenientWindowedResolutions(DisplayMetrics, Resolutions);

	return true;

Any idea how to solve this problem?