Unable to take screenshot in MacBook using shortcut shift+command+3/4
If the default location for saving screenshots has been changed, then this Terminal command will show you the changed location:

defaults read com.apple.screencapture location


If you get a message that the default pair does not exist, then the location has not been changed, and should be the Desktop. 

If you want to restore it to the default, enter:

defaults delete com.apple.screencapture location