With the Instant Pot lid off, set to SAUTE and spray the olive oil until Instant Pot says HOT. Add onion and garlic and cook until the onion is translucent and the garlic is fragrant.
Season the chicken breasts with salt and pepper and place in the Instant Pot and saute until browned. Add remaining chicken ingredients to Instant Pot. Securely lock the Instant Pot lid and set to MEAT setting for 13 minutes at HIGH.
Perform a quick release to release pressure when finished cooking. Carefully open the lid and shred the chicken in Instant Pot. Mix chicken with liquids.
Spoon 1/3 cup of the chicken mixture onto 6” tortilla. Top with lettuce, cheese, tomatoes and radishes or toppings of your choice. Serve immediately.