Preheat oven to 350. Prepare a 10" pan with cooking spray; set aside. In a small bowl, combine potatoes and 2 tablespoons taco seasoning mix. Press into bottom of prepared pan; set aside. In a skillet, heat oil over medium heat. Add chicken, onions, and bell peppers. Cook until chicken is no longer pink and vegetables are tender. Stir in beans, barbecue sauce, water, and remaining taco seasoning mix. Cook and stir until bubbly. Turn into crust. Bake, uncovered, for 30 minutes. Top with cheese and lettuce.