when a Blueprint class is created directly from AActor, it has a USceneComponent* Set as the Actors Root component.
When you create your own C++ class derived from AActor, you need to add a root component derived from USceneComponent, if you want to edit its transform.
Try Adding this to your constructor:
USceneComponent* Root = CreateDefaultSubobject<USceneComponent>(TEXT("Root"));
Root->SetMobility(EComponentMobility::Static);
SetRootComponent(Root);