Is zakat obligatory on a plot of land that its owner intended to invest in and then sell due to inability to invest after a year and a half of its purchase, and is she sinful for not having paid its zakat on time?

Zakat is not obligatory on the money with which the questioner bought the plot of land because a full year had not passed on it before the purchase. Zakat is also not obligatory on the land itself because it was not bought with the intention of trade, but rather with the intention of investing in it through agriculture. Her intention to sell it later when needed does not make it a trade asset, for the condition of trade assets is that they are bought with the intention of trade, not that the intention changes later.