How to buy skyliner tickets at Narita Airport at night?

That depends on how late you arrive. Narita is supposed to be a 24-hour airport. Unless the flight is delayed, there are few flights taking off and landing after 24 o'clock. Skyliner's ticket counter seems to open at 22: 30, and the last bus to Ueno, Beijing is also at 22: 30. If you can get off the plane and go through the immigration formalities, it is generally no problem to catch the last bus around 22: 00 after getting off the plane. You can only consider other JR routes or take the bus later.