Set the sauté function to high on a 6-quart or larger Instant Pot and add the butter. Once melted, add the sausage and cook, breaking up with a wooden spoon, until browned, about 6 to 8 minutes. Stir in the onion, celery, garlic, sage, thyme, salt and pepper. Cook until the vegetables have softened. Turn off the machine and transfer to a large bowl.
Add the chicken stock, stuffing mix and water chestnuts to the bowl and stir until combined. Transfer to a greased 7-inch round casserole or soufflé dish, pressing down slightly. Spray a sheet of foil with cooking spray and then cover the stuffing tightly with the foil, greased side down.
Clean out the bowl of the Instant Pot and then return it to the machine. Place the wire steaming trivet in the bowl and then add the water. Place the stuffing dish on the rack and the lock on the lid, making sure the valve is set to seal. Pressure cook on high for 12 minutes and then naturally release the pressure for 10 minutes before manually releasing any residual pressure inside.
Carefully open the lid and lift out the casserole dish. Remove the foil and serve immediately.
Tip: If you want to brown the top of the stuffing, you can place the dish under the broiler for about a minute to brown.