I guess you are using the latest version of the engine, which would make FPostConstructInitializeProperties deprecated.
It’s FObjectInitializer now.
The following code should be working:
//MyGameMode.h
UCLASS()
class THIRDPERSONTUTORIAL_API AMyGameMode : public AGameMode
{
public:
GENERATED_BODY()
AMyGameMode(const class FObjectInitializer& PCIP);
virtual void StartPlay() override;
};
//MyGameMode.cpp
AMyGameMode::AMyGameMode(const class FObjectInitializer& PCIP)
: Super(PCIP)
{
}
void AMyGameMode::StartPlay()
{
Super::StartPlay();
if (GEngine)
{
GEngine->AddOnScreenDebugMessage(-1, 5.f, FColor::Yellow, TEXT("HELLO WORLD"));
}
}