As the name implies ConstructorHelpers need to be in the constructor. You are invoking the command in the wrong place of your code, that is why it fails.
GameMode also doesn’t have a normal begin play anyway.
You need to call it from an actor’s CDO
If you have the class AMyActor then you need to call it in the cpp in
AMyActor::AMyActor(){
// -> call your function here.
static ConstructorHelpers::FClassFinder<AActor> BlueprintActor(TEXT("/Game/MyActor2_BP"));
if (BlueprintActor.Succeeded())
{
// rest of code
}
}