the first Free Play is unlocked after completing any of the main adventures that appear after the two tutorials. you must beat AREA 50 before resetting for it to count as a "completion". you could manually grind out this goal by resetting at 51 each time, or just let it come through natural progression. one of the most beneficial ways to progress through this game is by repeatedly resetting Free Plays, either earning favor, gems, or both. so you will very likely earn this along the way to earning other ones.