Preheat the oven to 425°F (220°C).
In a bowl, whisk together all herby ranch ingredients until smooth. Chill in the refrigerator.
On a large sheet pan, combine chicken strips, sliced peppers, and onion.
Drizzle with olive oil and season with garlic powder, paprika, oregano, cumin, chili flakes, salt, and pepper. Toss well.
Spread everything in a single layer.
Roast for 20–25 minutes, flipping halfway, until the chicken is fully cooked and vegetables are tender.
Warm the pitas.
Assemble the Sheet Pan Chicken Pitas with Herby Ranch by filling each pita with lettuce, chicken, vegetables, tomatoes, and a generous drizzle of sauce.