Did you recently install any rootkit-level software on your Mac? I experienced this after taking an AWS exam on Pearson OnVue. Any overly privileged downloaded can cause this. If you did download some application recently, it probably broke some system preferences that can't be fixed without some effort.
- Open Terminal
- Type (without quotes): "defaults delete ~/Library/Preferences/com.apple.coreservices.useractivityd.plist ClipboardSharingEnabled"
- Type (without quotes): "defaults write ~/Library/Preferences/com.apple.coreservices.useractivityd.plist ClipboardSharingEnabled 1"
It should work at this point. If it doesn't restart your computer, try again/
See this discussion thread for more details or get help from Apple Support