Learn Russian: Клиент ищет просторный участок, на котором мы можем строить. - The client is looking for a spacious plot on which we can build.

который (which/who) changes its ending to match the noun it refers back to, just like an adjective. Here it refers back to участок (plot). Since we are talking about building on the plot, we use the preposition на followed by the Prepositional case for a masculine noun: на котором.

Why do we use the preposition на here instead of в?

The word участок means a plot or piece of land. Because it is an open outdoor area rather than an enclosed space, Russian uses на (on) to say 'on the plot' instead of в (in).

I don't recognize the verb ищет. What is its dictionary form?

It comes from the verb искать (to look for / to search). This verb has a tricky stem change in the present tense where the ск turns into a щ. So он ищет means 'he is looking for'.

The client is looking for a plot, so shouldn't просторный участок be in the Accusative case?

They are in the Accusative case! The direct object of ищет is a masculine, inanimate (non-living) noun. For inanimate masculine nouns and their matching adjectives, the Accusative form is identical to the Nominative (dictionary) form.