Back Camera not function after changing display

Your phone is telling you that it is too hot to use the flash. This could be caused by something pulling excess current, hence creating heat, in the area near the temperature sensor.

Have you noticed any excess heat on the front or back of the device? You said you changed the LCD and I assume it was because it was cracked. Can you put back the old screen and see of it still gives you the same message? If not, then something on the replacement screen is pulling too much current.