Set an Instant Pot to Sauté, and heat oil for 2 minutes. Add beef to hot oil, and stir to break into pieces; cook, stirring occasionally, until browned, about 8 minutes. Add onion; cook, stirring once, until onion is starting to soften, about 6 minutes. Add flour, taco seasoning mix, and ranch mix; cook, stirring occasionally for 2 minutes.
Stir in stock, corn, tomatoes, and beans. Seal Instant Pot with lid, and set steam vent to seal. Change settings to Manual Cook on high pressure, and set timer for 2 minutes (Instant Pot will take at least 10 minutes to build pressure before counting down). Carefully release steam valve. Once steam is released, open lid and divide soup evenly among 6 bowls. Top each serving with cheese, sour cream, and scallions.