Copy-Paste not working after macOS Tahoe upgrade

Test in Safe Mode – Restart your Mac and hold the power button until you see “Options”, then select your user and hold Shift to boot Safe Mode. Does the issue persist?

Test in a different user account – If you don’t already have one, create a new user in System Settings → Users & Groups, log into it and test copy-paste there.

If it works fine for the new user, likely it’s something in your main user account (settings, prefs, extension).

Keyboard shortcut conflicts – Go to System Settings → Keyboard → Keyboard Shortcuts and see whether ⌘ C / ⌘ V have been accidentally remapped or disabled.

Third-party utilities – Do you have any clipboard manager, hot-key remapper, accessibility tool, macro utility, etc.? These could interfere with standard shortcuts. Try quitting them (or disabling) and see if it resolves.

Check for stuck modifier key or keyboard error – Press ⌘ C, ⌘ V in a simple app (TextEdit) and see if the window closes, or if only a specific app behaves this way. If it’s app-specific maybe the app intercepts those keys.