Why isn't my iPhone 6s Plus cameras working ?

Hello,

Where did you get the repair at? An Apple Store? An Apple Authorized Service Provider?

You see a closed lens or black screen

If you see a closed lens or black screen when you open the Camera app, try these steps:

1. Make sure that there’s nothing blocking the camera lens. If you’re using a case, try removing it.
2. Force the app to close, then open the Camera app again.
3. Restart your device, then open the Camera app again.
4. If your device has a front and rear camera, try both cameras by tapping . If you see the closed lens or black screen on only one camera, take your device to an Apple Retail Store or Authorized Service Provider for more help.

More info here:

Get help with the camera on your iPhone, iPad, or iPod touch - Apple Support

Okay if the above steps didn't help, I would suggest taking your iPhone back to the Apple Authorized Service Provider you got the repair from. And explain your situation and they could help guide you in the right direction.

Whoever fixed it did a poor job. I hope it was Apple, because if it wasn't Apple will not longer touch that phone. But the only solution is to take it back to whoever fixed it.