In a medium bowl, whisk together olive oil, lemon juice, minced garlic, oregano, paprika, salt, pepper, and red pepper flakes until well combined.

Add chicken breast to the marinade, ensuring each piece is coated. Cover with plastic wrap and let marinate for 15-45 minutes.

Combine cucumbers, tomatoes, bell pepper, red onion, and parsley in a separate bowl. Add feta cheese, lemon juice, salt, and pepper. Mix gently and let it sit.

Heat a skillet over medium-high heat. Add marinated chicken in a single layer. Cook for 3-5 minutes on one side, then flip and cook for another 3-5 minutes or until cooked through.

Warm pitas over a gas flame for 10-15 seconds on each side or in a skillet over low heat for 1 minute.

Assemble tacos by placing chicken in each warm pita. Top with salsa and drizzle with tzatziki.