Season the chuck roast. Pour broth into Instant Pot and add the chuck roast. Pressure cook 70 minutes. When time is up move the valve to venting. Open and place a steamer basket on top of the beef.
Add the potatoes to the basket. Pressure cook 8 minutes. Let sit for 15 minutes then vent. Dump the potatoes into a large bowl, add in the butter, milk, salt, garlic powder and sour cream and mash until smooth.
Move roast to a cutting board. Strain off the fat from the broth. Add strained broth back into the Instant Pot. Turn the Instant Pot to sauté and whisk in the gravy packet. Shred the beef.