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.