Q: Truck won't start, but lights come on and flicker

What you are describing sounds like a faulty starter solenoid. A faulty starter solenoid will generally produce a clicking sound when turning the key. The starter solenoid is located on the starter and has internal electrical contacts that transmit electricity to the starter. As electricity is supplied to the starter a small gear comes out and engages the flywheel to spin the engine over as you turn the key. When this fails, it may cause a clicking sound and will not allow the starter to engage the flywheel when you turn the key. In some cases if the starter solenoid is faulty it may cause the starter gear to remain out too long which can cause the starter gear to grind against the flywheel after the engine has started. This is noticeable by a very loud grinding sound. I would recommend having a professional from YourMechanic come to your location to diagnose and inspect your vehicle.