Build errors?

It looks like you haven’t defined the BeginPlay() function correctly in FPSGameMode.cpp.

From what I can see looking at the tutorial, your body should look like this

AFPSGameMode::AFPSGameMode(const class FPostConstructInitializeProperties& PCIP)
	: Super(PCIP)
{
	
}

void AFPSGameMode::BeginPlay()
{
	Super::BeginPlay();

	if (GEngine)
	{
		GEngine->AddOnScreenDebugMessage(-1, 5.f, FColor::Yellow, TEXT("HELLO WORLD"));
	}

}

See if your code matches. It’s possible you’ve defined the BeginPlay() function inside the class constructor.