Dark Knight is part of the Heavensward expansion. Since FFXIV free trial encompasses it, you do not need to purchase the game if you want to sample the Job. However, it's necessary to finish all the base game content, A Realm Reborn, up until the MSQ "Before the Dawn," before you can access the Foundation, a main hub of Heavensward where the NPC Ishgard Citizen is located, in The Pillars (X:13.2 Y:8.8), which starts the sidequest to become a Dark Knight.