I set camera as the child of a ball, when I press arrow key, it starts rotating, but ball don't.

I set camera as the child of a ball, when I press arrow key, it starts rotating, but ball don’t. Why ?
I followed the roll-a-ball tuturial step by step. My code’s fine. is there something wrong with that tuturial ? I just want the camera moving horizontally. Help me!!!

creating an empty gameObject with both is a good way