That’s a good question that hopefully someone more learned will be able to respond to but you know most of the Earth’s surface is water so I suppose Neptune has a surface. I don’t know what the technical definition is but I’m assuming it’s where particulate / molecules in the atmosphere change states due to temperature and pressure. A bit like how we have H2O in the form of gas as water vapor in the atmosphere and at the surface a liquid as water. The closer you are to the center of the planet the higher the pressure and the temperature.