Set the Instant Pot to "Sauté" mode and heat the oil. Add the cubed chicken and sear until golden brown. Remove chicken and set aside.
In the same pot, add onions and garlic. Sauté until softened. Incorporate carrots, peas, green beans, and red bell peppers. Sauté for a few minutes until vegetables start to soften.
Stir in tomato paste and diced tomatoes. Cook for an additional 2 minutes.
Return the seared chicken to the pot. Add potatoes, salt, oregano, and black pepper. Mix well to combine. Pour in chicken broth, ensuring all ingredients are submerged. Cancel "Sauté" mode. Secure the Instant Pot lid, set to "Pressure Cook" mode, and cook for 8 minutes.
Allow for a natural release for about 10 minutes, then carefully perform a quick release to release any remaining pressure. In a small bowl, mix cornstarch with a bit of water to create a slurry. Stir the slurry into the stew, then set the Instant Pot back to "Sauté" mode. Cook until the stew thickens to your desired consistency.
Add the olives to the stew, taste and adjust seasoning if necessary. Serve the Instant Pot Chicken Stew hot, garnished with fresh herbs if desired.