Why don't I have a left turn signal and brake lights
No brake lights or turn signal
Also sold under the GMC Sierra and VIA Vtrux names, available in the US, Canada, Mexico and other countries around the world.
Let us know the exact year of your truck. Of course you already checked the bulbs and those are good. Right? It sounds like you have no power or ground to the complete light housing. There should be connector leading away from it (remove the light), unplug it and see if you have power going to the connector (bussing blocks). If so you know that the error is in your light assembly.
when in doubt or if you get stuck somewhere post some good images with your question. That way we can see what you see and can hopefully provide you with more help. Let us know what you find out.