You can only use the died event on an Instance if its a humanoid so zombie.Humanoid.Died and FireClient assuming the script is a local script you would have to pass in the player as a parameter.
Hi guys, sorry for the 11 days that I didn’t reply. I have been looking through tutorials / free script that I can use for a base of my level system. I finally find one recently (Actually now) and I decided to use it. Since the script is using clicks for gaining XP, I will modify the script to apply for my situation. Also is it ok that your guys can give me a basic formula for my level system?