In a small bowl, combine yogurt, lemon juice, salt, and pepper.
In a medium bowl, toss apple, celery, cranberries, and walnuts. Add the yogurt mixture and stir to combine.
On each tortilla, lay 1 lettuce leaf, ½ cup chicken, and ¼ cup of the apple mixture. Roll sides of tortilla inward and roll away from you to form a cylinder. Cut in half.