Set the Instant Pot to sauté. Add the ground beef and onion. Cook, breaking up the meat, until the beef is no longer pink.
Stir in the garlic, Italian seasoning, salt, and pepper. Cook for one minute until fragrant.
Add the carrots, green beans, barley, diced tomatoes with their juice, and beef broth. Stir well to combine.
Secure the lid, set the valve to sealing, and cook on high pressure for 20 minutes.
Let the pressure release naturally for 10 minutes, then quick release any remaining pressure. Stir the stew before serving.