Screenshot High sierra 10.13.6

First try restarting your Mac. If that doesn't work, check System Preferences > Keyboard > Shortcuts > Screenshots and make sure the shortcut is enabled.

Does 4+shift+kommand+ctrl allow copy of a section of the screen to the copy buffer?

What happens when you try, do you hear the camera click? Maybe the save location has been changed?

Also try using Grab in your Applications/Utilities folder.

You're welcome.