Is zakat due on land that a father bought with the intention of trading it, and then sold it after 15 years, knowing that he did not pay zakat on it due to ignorance of the ruling, and he passed away before paying the zakat?

Zakat (definition) is not obligatory on land unless three conditions are met: it is purchased with the intention of trade, its value reaches the nisab (definition) (minimum threshold), and a full year (hawl) passes on the money with which it was purchased. If these conditions are met, Zakat becomes obligatory for each year, even if the land has not been sold. In the event that these conditions are met for the land mentioned in the question, its Zakat becomes obligatory for all past years by estimating its value each year and paying a quarter of a tenth (2.5%) of the value, based on the Almighty's saying: ﴿Allah does not burden a soul beyond its capacity﴾.