How can I make a variable with the type of an AActor?

Try this;

UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "City")
 TSubclassOf<AActor> BuildingInstanceActorClass;
 
 void Populate();
 
 FBuildingBlueprint()
 {
     BuildingInstanceActorClass = ABuildingInstanceActor::StaticClass();
 }

The only change is TSubclassOf instead of UClass*.