You need to be more specific on the type of turtle. But regardless of the species, the answer is no. Lettuce has barely any nutritional value. It's mainly made of water.
Water turtles (sliders, mud turtles, and terrapins) eat mainly live food in the wild. In captivity they eat fish, worms, and crickets. You can also feed them commercially available turtle pellets along with occasional live food.
Box turtles and other land tortoises feed on a variety of vegetables, fruits, and insects. Most greens such as collard greens, mustard greens, and kale are healthy. They will eat fruits like bananas, cantaloupe, and apples. They like insects like mealworms and earthworms.
If you found the turtle in the wild, please let it go. Turtles taken from the wild never do as well in captivity. Captivity will stress them out and often kill them. They also get very large and are difficult to care for properly. Unless you want to purchase an 80-100 gallon tank, heat lamps, filters, and uvb lights, do not keep a turtle. They are not pets like fish or hamsters.