First, check System Preferences > Keyboard > Keyboard Shortcuts to see if cmd-shift-N has been mapped to anything. Be sure to check all the system categories as well as Application Shortcuts > All Applications.

If that doesn't reveal the culprit, you might want to look to see if you have any "Input Manager hacks" installed. These are third-party tools that use a skanky method to modify the behavior of the system and other apps. Some of the best known Input Manager hacks include SIMBL (and tools that use SIMBL, such as Afloat) and APE (Unsanity's Application Enhancer, and related "Haxies"). If you find any input manager hacks, remove them according to their instructions.

Make sure when you press CMD+C your Edit menu in the top flashes, this indicates it works as expected.

If it doesn't, go to Keyboard settings in System Preferences and:

• Restore defaults Modifier Keys...,
• Restore defaults in Shortcuts tab,
• In App Shortcuts click the + button and test if Keyboard Shortcut input box will react on your combination.

If Edit menu flashes and it still doesn't work, then the first thing to do is to restart pasteboard service by the following commands:

launchctl stop com.apple.pboard
launchctl start com.apple.pboard

If still doesn't work, here are some other suggestions to troubleshoot the problem:

• Re-test in different applications just in case.
• Re-test if works when using contextual menu (without using the shortcut).
• Re-test alternative combination such as CMD+X for Cut instead.
• Run Console to see your System log queries if something obvious is happening.

• Test if the Clipboard works from the terminal via:

  $ echo test | pbcopy
  $ pbpaste
  

  If Paste menu item is no more greyed out, then maybe it's a problem with Paste command.

  If you're advanced user, use dtruss or fs_usage to debug it, for instance:

  $ sudo fs_usage | grep -C4 MyAppWhereCopyDoesntWork
  

  and then notice other apps which appearing exactly when you press the shortcut which potentially you could consider quitting for testing purposes.

• Disable any recent programs that you run or restart your computer.

• Disable VoiceOver as suggested here.
• Force quit the Finder (e.g.: killall -HUP Finder).
• If still doesn't work, consider rebooting the system, especially in Recovery or Safe mode to re-test.

As for a workaround, you can install Karabiner app and remap your shortcuts by configuring private.xml file (see Misc tab). For more suggestion, please check the following posts:

• Remap Ctrl+C, Ctrl+V, and Ctrl-X only
• How can I reassign the copy/paste keyboard shortcuts?

It seems you need to have a folder selected in Finder to make it work, it won't if you've got an empty selection or a file selected.

Also make sure that your keyboard shortcut is not used by any other function, it doesn't warn about it. (I tried CMD+CTRL+X, which seemed to work, but had trouble with many other combinations.)

These are symptoms of a non-functioning Command key. Can you try using your right-most Command key as well to test with Command + space and Command + v? If it works with the right Command button, then that means your left Command key somehow has broken, indicating keyboard failure, not uncommon with Intel Macs from around 2016-2020 sad to say, when the infamous "butterfly keyboard" was being distributed on Macs in those years.

But if the right Command key doesn't work, then there's probably something fishy going on with some setting somewhere. I found this Command Key Not Working post, maybe it might help.

I found this other site, saying if you go System Preferences -> Keyboard -> click on Modifier Keys..., make sure Command key has a function.