Preheat the oven to 350 degrees F, for 3-5 minutes.
In a large bowl, mix soup, milk, chicken, salt, and pepper. Spread the mixture into a deep casserole dish that fits into your air fryer.
Cook in the air fryer, covered with aluminum foil, at 350 degrees F for 20 minutes.
Meanwhile, cook 4 tablespoons of butter, onions, and celery over medium heat on the stovetop. Cook until the vegetables are softened and translucent (about 8 minutes).
Add the entire contents of the stuffing package to the skillet. Pour water over the stuffing and stir to mix everything together.
Remove the soup mixture from the air fryer and stir to distribute the heat evenly. Top with the prepared stuffing/vegetable mixture.
Return the casserole dish to the air fryer and cook, uncovered, for an additional 10-15 minutes. Casserole is done when it is heated through and the top is browned.
Melt the remaining two tablespoons of butter and drizzle over top, if desired.
Serve immediately.