I have a 2000 Silverado 4.8 v8 when starting it feels like the battery is dead I have a optima red battery on it I have charged it and check the voltage it reads 11.9v any idea what it could be

It should read around 12.5v or better.

It's not about volts. You need amps. 11.9 still isn't charged. Take it to a parts store and have them load test it

The test station should do a load test. When you say it feels like the battery is dead, do you mean it barely turns over the engine? Sounds like a bad cell in the battery or the charging system is not working as it should. Make sure the battery connections are clean and connected/tight. If you have oil/grease/dirt/corrosion on the starter connections, that could prevent a full electrical load from getting through. Think of it like a water hose. Step on a running water hose and the water will barely get through. A low battery will be like the hose, water will trickle through but the water turn on valve is not fully opened.