I get an error sound when trying to take a screenshot
Ever since I had to use the website ProctorU to take an online test I haven't been able to take screenshots. I use "command, shift, 4" and that's always worked until now. I don't get an error message just the 'bum' sound when you can't do something. Not sure how to fix this. Thanks in advance
MacBook Pro with Retina display, OS X El Capitan (10.11.3)