To be able to turn a NeoPixel LED on or off, you must first initialise the NeoPixel class. This means that you are telling MakeCode that a number of Neopixles exist and that they are Red, Green, Blue (RGB) responsive. Once you have done this, you can then call the show method on this class.

This video shows the process of initialising a Neopixel in the Makecode editor.