Trouble with first part of Experiment 11 (Make: Electronics)
It's hard to tell from the photo, so I may be incorrect, but it appears that the "G" pin on your PUT is connected to the breadboard, but that nothing is connected to a rail that it shares. It may sound like the obvious, but it helps to double check that everything is firmly connected to the board and that the board is being powered reliably (I.E. No dead batteries, etc.). More often than not, it's simple mistakes that can make a circuit not work no matter what you try. It won't hurt to double check that you have your capacitor and LED polarities correct too.