Combine flour and 2 tablespoons of the fajita seasoning in a large zip-loc bag. Add cubed chicken and give the bag a good shake to coat the chicken.
Set Instant Pot to Sauté setting. Add olive oil, and when hot, add the chicken to the pot. Sauté, stirring occasionally until chicken is lightly browned (Some of the flour coating will probably stick to the bottom of the IP. This is normal, and will be fixed in a minute).
Add onions and sauté until soft, about 2 minutes, then add the garlic and cook an additional minute. Add the chicken broth to the pot and use a spoon or spatula to help deglaze the bottom of the pot (i.e scrape the bottom of the pot with your spoon to release those browned bits).
Add the remaining fajita seasoning, corn, black beans, stewed tomatoes, green chiles, and rice to the pot.
Put the lid on the IP and switch it to manual. Set timer for 10 minutes at high pressure. When the time is complete, do a quick pressure release. Stir in nacho cheese soup. Serve with desired garnishes.