Rub both sides of chicken breasts with taco seasoning mix. In 12-inch nonstick skillet, heat oil over medium-high heat until hot. Cook chicken in oil 8 minutes, turning once. Decrease heat to medium; cover skillet. Cook 5 to 8 minutes or until thermometer inserted in thickest part of the chicken reads 165°F. Let stand 3 minutes. Thinly slice chicken.
Heat tortillas as directed on package. Fill tortillas with chicken, chiles, avocado and sour cream.