As another discussion on this lecture pointed out if you are following along exactly as Sam does there is a part where in the struct FGoKartState there is a missing UPROPERTY() above FGoKartMove LastMove. Code below is what it should look like. So if you are like me and trying to follow the code exactly then this part gets missed.
USTRUCT()
struct FGoKartState
{
GENERATED_BODY()
UPROPERTY()
FVector Velocity;
UPROPERTY()
FTransform Transform;
UPROPERTY()
FGoKartMove LastMove;
};