EDIT: PROBLEM SOLVED. I just forgot to add "return total_cost" at the end. Sorry for the inconvenience. Topic can be locked.
Oops, try again. It looks like rental_car_cost returns None instead of the correct amount (40) for 1 days.
def rental_car_cost(days): total_cost = 40 * days if days >= 7: total_cost -= 50 elif days >= 3: total_cost -= 20