Place oats in a multi-functional pressure cooker (such as Instant Pot®). Sprinkle with pumpkin pie spice and salt. Add water, milk, pumpkin puree, and cranberries. Stir lightly.
Close and lock the lid. Set to Manual; select high pressure according to manufacturer's instructions. Set timer for 3 minutes. Allow 10 to 15 minutes for pressure to build. Keep warm for 5 minutes more after cooking time is complete.
Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove lid. Stir in honey; divide oatmeal into 4 bowls. Top each serving with 1 tablespoon walnuts and 1 1/2 teaspoons brown sugar.