Add the potatoes to the bottom of your instant pot. Pour the water on top.
In a large bowl combine beef, onion, parsley, Italian breadcrumbs, milk, Worcestershire sauce, eggs, garlic powder, salt and pepper. Use your hands to mix until combined being careful not to overmix.
Place a sheet of aluminum foil on the counter. Shape the meatloaf into a loaf and shape the sides of the aluminum foil around the meatloaf. Place on top of the potatoes in the instant pot.
In a small bowl whisk together ketchup, vinegar and brown sugar. Brush half of the mixture on the top of the meatloaf.
Cover and turn the instant pot to the sealing position. Pressure cook on manual high for 25 minutes. Release the pressure with the quick release. Check the meatloaf to see if it has reached an internal temperature of 155 degrees.
Remove the meatloaf from the pan and let it rest.
Add the milk and butter to the potatoes and mash with a beater or hand masher until they reach the desired consistency. Salt to taste.