Turn on a multi-functional pressure cooker (such as Instant Pot) and select Saute function and allow pot to heat. Add butter and melt. Add sausage and chopped onion. Stir until sausage is crumbled and onions are browned, about 5 minutes. Stir in pumpkin and 1 cup of chicken broth. Add remaining 2 cups chicken broth, 1 cup at a time, stirring well after each addition. Close and lock the lid. Cancel Saute function.
Select Soup setting according to manufacturer's instructions; set timer for 10 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid. Stir in cream, salt, sage, and garlic. Serve immediately.