Char is a nil value?

The character doesn't load instantly after the player sometimes, so you have to wait for it to load. Use this method:

local char = plr.Character or plr.CharacterAdded:Wait()

That way you know char variable isn't ever going to be nil.

Also, I suggest using WaitForChild for the Humanoid so you know that's not nil as well.

local Player = game.Players.LocalPlayer
local char = Player.Character or Player.CharacterAdded:Wait()
local Humanoid = Char:WaitForChild("Humanoid")

Another thing I highly suggest is NOT using KeyDown. Use UserInputService instead.

local Player = game.Players.LocalPlayer
local char = Player.Character or Player.CharacterAdded:Wait()
local hum= char:WaitForChild("Humanoid")
local lastUse = 0;



local function onKeyDown(inputObject) 
    local key = inputObject.KeyCode
    if key == Enum.KeyCode.Z and Player.Levelss.Level.Value >= 15 and tick()-lastUse > 10 and Player.Levelss.RangedPunch.Value < 1  then 
        local x = Instance.new("Part")
        x.BrickColor = BrickColor.new("White")
        x.Material = "Neon"
        x.CanCollide = false
        x.Transparency = 0.6
        x.Size = Vector3.new(10,10,10)
        x.TopSurface = "Smooth"
        x.BottomSurface = "Smooth"
        x.Shape = "Ball"
        x.Name = Player.Name
        local fd = script.fireDamage:clone()
        fd.Parent = x
        local a = Instance.new("Sound")
        a.SoundId = "http://www.roblox.com/asset/?id=260430117"
        a.Parent = x
        a.Volume = 2
        a:play()


        Player.Levelss.XP.Value = Player.Levelss.XP.Value + 8


        local y = Instance.new("BodyVelocity")
        y.maxForce = Vector3.new(math.huge ,math.huge ,math.huge)
        y.velocity = Player.Character.Torso.CFrame.lookVector*95

        x.CFrame = Player.Character.Torso.CFrame*CFrame.new(0, 1 , -10)
        x.Parent = Workspace
        y.Parent = x    
        game.Debris:AddItem(x, 6)

        lastUse = tick(); 
    end
end


game:GetService("UserInputService").InputBegan:Connect(onKeyDown)

This is only an example.