Turn instant pot to sautee setting.
Add oil, heat until shimmery.
Add pot roast, let brown on each side (about 6 minutes per side).
Remove pot roast, set aside.
Add celery, carrots to pan.
Cook until soft and lightly browned, about 5 minutes on sautee.
Deglaze with stock.
Add roast back into pan.
Add ranch dressing packet to season.
Add lid to instant pot, set to seal.
Cook on meat with high pressure for 38 minutes, and let naturally release.
Remove pot roast from pan.
Set instant pot to sautee and reduce sauce to a thick gravy.
Pour gravy over roast and serve immediately.