"Do I mark $0 for dependent care then?"
For which question?
If you had dollars in your FSA at the end of 2020 and you now realize that your employer has modified the FSA plan to allow you to carry it all over to 2021, then in the W-2 interview, please enter the full amount that was unspent at the end of 2020.
If, however, you are asking something else, please come back and tell us.