Start by cooking your chicken breast. You can boil, grill, or bake it until fully cooked. Once done, allow it to cool slightly, then shred it using two forks or your hands.
While the chicken cools, chop the celery into small pieces and halve the grapes.
In a mixing bowl, combine the Greek yogurt, Dijon mustard, salt, and pepper. Stir until smooth and well mixed.
Add the shredded chicken, chopped celery, and halved grapes to the bowl with the dressing. Gently mix until all the ingredients are well coated in the dressing.
Lay out your tortillas on a clean surface. Place a few lettuce leaves in the center of each tortilla, followed by a generous scoop of the chicken salad mixture.
Fold in the sides of the tortilla, then roll it from the bottom up to create a tight wrap. Repeat with the remaining tortillas and chicken salad.
Slice the wraps in half for easier handling, and serve immediately.