Is zakat obligatory on the amount from selling a car to one's father in installments, especially if the received amount has not yet reached the nisab? And is zakat paid on the amount received or on the entire debt? And is zakat obligatory every year if the installment payment period is prolonged?

You must pay Zakat (definition) on all the money your father owes you, because it is your property. Zakat becomes due on it when a full year has passed, even if you have not yet received it. However, since your father is in financial difficulty, you should pay Zakat on the money you have received when a full year has passed since its receipt, and pay Zakat on the remainder when you receive it, for every year in which Zakat was not paid. It has also been said that you pay Zakat on it for one year when you receive it.