In order to unlock Dancer, you must be at least Level 60 on one Disciple of Magic or War job and have access to the Shadowbringers Expansion. Once you have a Disciple of Magic or War at level 60, you will head to Limsa Lominsa Lower Decks (x:9.8, y:12.0) and pick up the quest 'Shall We Dance' from the NPC named Eager Lominsan. Upon completing this quest, you will be rewarded with the Dancer job stone from Nashmeira and start out as a Level 60 Dancer.