Q: System preferences won(t close down after canceling an account

Hello JEHAN1,

Thanks for that info and choosing the Apple Support Communities. I know that having an issue with System Preferences on your Mac not closing correctly is not what you expect!

If I understand correctly, you deleted a user on your Mac that is no longer being used. Then when you quit System Preferences, System Preferences stopped responding and will not close. The good news is you have a couple of options:

1. First, you can try to use the Force Quit option in the Apple logo menu to force System Preferences to Quit, then restart your Mac.
 

Force an app to close on your Mac

2. If you're still not able to get System Preferences to close, then shut down your Mac by choosing Shut Down from the Apple menu, or hold down the power button for up to 3 seconds until your Mac asks whether you're sure that you want to shut down your computer now. 
 

How to turn on or turn off your Mac

3. If you can't shut down your Mac because it isn't responding, you can force it to turn off without shutting down. Hold down the power button for 5 seconds until your Mac turns off. Use this method only when you can't shut down.

You also may want to try starting up into Safe Mode to help resolve this, as Safe Mode performs certain system checks, deletes systems cache files, verifies your startup disk, and attempts to repair directory issues, if needed.
 

Try safe mode if your Mac doesn't finish starting up

Cheers!