Marinate the chicken by whisking together olive oil, lemon juice, garlic, oregano, paprika, salt, pepper, and red pepper flakes. Cut chicken into bite-sized pieces and coat in marinade. Let sit for 15-45 minutes.

Prepare the salsa by combining cucumbers, tomatoes, bell pepper, red onion, parsley, and feta. Drizzle with lemon juice and season with salt and pepper. Let sit for 10 minutes.

Cook the marinated chicken in a preheated skillet over medium-high heat for 3-5 minutes until golden brown, then flip and cook for another 3-4 minutes until the internal temperature reaches 165°F.

Warm the pitas over a gas flame or in a dry skillet for about 30 seconds on each side. Alternatively, wrap in foil and heat in a preheated oven at 350°F for 5 minutes.

Assemble tacos by filling each pita with chicken, topping with salsa and tzatziki sauce. Serve immediately.