Heat a large skillet over medium-high heat. Add the ground chicken and cook, breaking it into small pieces with a spatula, until fully cooked through and no pink remains, about 6–8 minutes.
Drain and discard any excess fat from the skillet. Return the skillet to the heat, add the taco seasoning and 1/4 cup water, stir to combine, and cook until most of the water is absorbed, about 1–2 minutes.
Warm the tortillas by stacking them with a slightly damp paper towel between each and microwaving until heated through, about 45 seconds.
Divide the seasoned chicken evenly among the 6 tortillas. Top each with diced tomato, chopped romaine, shredded cheddar, and a dollop of Greek yogurt (or sour cream).
Fold the tortillas over the fillings and serve immediately.
