Wiring diagram

An LED never lights up

LEDs have a direction, need a resistor, and must match the pin in the sketch.

The problem

The traffic light (or any LED in a package) stays dark on one or all colours.

Polarity: LEDs only pass current one way. The longer leg is usually positive (toward the pin). The shorter leg goes toward GND.

1
Swap the LED around. If it was backwards, it will light on the second try. This is the most common first-time miss in QA.
2
Use a resistor (about 220 ohm) in series. A naked LED on 5V can burn out instantly and then look like a wiring fault forever.
3
Confirm digitalWrite in the code uses the same pin you plugged into. Traffic-light QA uses D8/D9/D10. A wire in D7 will never light from that sketch.
4
Test the LED on a known-good pin that you set HIGH in a two-line sketch. If it still fails, replace the LED.

If you walked the steps and the mechanism still does not move, write to us. This is the fallback, not the first stop.