There are several reasons why your 2012 Sierra truck with a 5.3L engine, 6-speed gearbox, 3.42 gearing, Goodyear Eagle LS-2 275/55/R20 tires, custom tune, real dual exhaust, and a KN CAI will not do a burnout.
It might first be due to the tires. If you are using all-terrain tires, they may not have sufficient traction to prevent tire spin when the gas pedal is pressed to the floor. Because all-terrain tires are intended with a more aggressive tread pattern to tackle off-road conditions, they may not contain sufficient rubber to allow the tires to spin.
Second, your truck might not have sufficient power to free the tires. Even with the custom tune, real dual exhaust, and KN CAI, your 5.3L engine’s torque output may not be sufficient to separate the tires. This is especially the case if your gear ratio is low.
Finally, it might be an issue with the traction control. Your truck’s traction control system may be overly severe, preventing the tires from spinning. This system is intended to avoid tire spin and retain vehicle control in slippery weather, but it may be too strong to enable tire spin.
If you wish to perform a burnout, you should begin by ensuring that the tires you are using are built for optimum traction. Then, you must ensure that your vehicle is tuned and geared to create sufficient power to separate the tires. To enable the tires to spin, you may need to disengage your traction control system.