Place chicken between 2 sheets of plastic wrap; pound to 1/2-inch thickness using a meat mallet or small heavy skillet. Combine paprika and next 7 ingredients (through pepper) in a small bowl. Sprinkle paprika mixture evenly over chicken; let stand 10 minutes.
Combine juice and avocado in a medium bowl, tossing to coat. Heat a grill pan over medium-high heat. Coat pan with cooking spray. Grill avocado 2 minutes on each side or until well marked; remove from pan.
Place chicken in pan. Cook 4 minutes on each side or until done. Let chicken stand 5 minutes; cut chicken crosswise into (1/4-inch-thick) slices.
Warm tortillas according to package directions. Divide the chicken evenly among tortillas; top each with 2 avocado slices, 1/4 cup lettuce, 2 tablespoons salsa, 1 1/2 teaspoons sour cream, and 1 1/2 teaspoons onion.