I'm unable to take screenshots using the print screen button , and when i tried changing the settings also noticed you can't assign single key for the same , this used to work perfectly in freya . p.s i had to take the picture from a external device
It doesn't work for me either. I do not know if it is a bug or desired behaviour. Best thing to do would be to open up a bug report. This will bring the issue to elementary OS's team attention.
Meanwhile you can use the internal screenshot app of elementary OS: Open the applications menu on the top left side of the screen and type screenshot. You can then choose to make a picture of the whole screen or port of it.