Is zakat obligatory on agricultural land that was purchased 15 years ago with the intention of building on it or selling it, but it has not been possible to sell it or build on it?

Land bought for building and living in is not subject to Zakat, because it is not for trade. The mere intention to sell it when its price rises does not obligate Zakat unless it is sold and its price is received and a hawl (full lunar year) passes over it.