Why does it say Attempt to index nil with kills?

So I wanted to make it, that if someone clicks the click detector, then it checks to see if the part touched the humanoid and killed it. But for some reason, it says attepmt to index nil with kills.

Here are the scripts.

Leaderboard.

game.Players.PlayerAdded:Connect(function(plr)

	local stats = Instance.new("Folder", plr)
	stats.Name = "leaderstats"

	local kills = Instance.new("IntValue", stats)
	kills.Name = "Kills"

end)

And the cannon script i modified to my needs(check the first if statement)

ball = script.Parent
damage = 50

function onTouched(hit)


	local humanoid = hit.Parent:findFirstChild("Humanoid")
	if humanoid~=nil then
		tagHumanoid(humanoid)
		humanoid.Health = humanoid.Health - damage
		if humanoid.Health == 0 or humanoid.Health < 0 then
			local z = game.ReplicatedStorage.Value.Value
			z.leaderstats.Kills.Value += 1
		end
		wait(2)
		untagHumanoid(humanoid)
		connection:disconnect()
	else
		damage = damage / 2
		if damage < 2 then
			connection:disconnect()
			ball.Parent = nil
		end
	end

	if math.random(1,3) == 2 then
		explosion = Instance.new("Explosion")
		explosion.BlastRadius = 9
		explosion.BlastPressure = 1000000 -- these are really wussy units
		explosion.Position = script.Parent.Position
		explosion.Parent = game.Workspace
		connection:disconnect()
		ball.Parent = nil
	end

end

function tagHumanoid(humanoid)
	-- todo: make tag expire
	local tag = ball:findFirstChild("creator")
	if tag ~= nil then
		local new_tag = tag:clone()
		new_tag.Parent = humanoid
	end
end


function untagHumanoid(humanoid)
	if humanoid ~= nil then
		local tag = humanoid:findFirstChild("creator")
		if tag ~= nil then
			tag.Parent = nil
		end
	end
end

connection = ball.Touched:connect(onTouched)

r = game:service("RunService")
t, s = r.Stepped:wait()
d = t + 5.0 - s
while t < d do
	t = r.Stepped:wait()
end

ball.Parent = nil

All help appreciated

local z = game.Players:FindFirstChild(game.ReplicatedStorage.Value.Value))

Answering behalf of greatneil

Replace your canon script with this

ball = script.Parent
damage = 50

function onTouched(hit)


    local humanoid = hit.Parent:findFirstChild("Humanoid")
    if humanoid~=nil then
        tagHumanoid(humanoid)
        humanoid.Health = humanoid.Health - damage
        if humanoid.Health == 0 or humanoid.Health < 0 then
	    local player = game.Players:FindFirstChild(hit.Parent.Name)
            local z = player.leaderstats
            z.Kills.Value += 1
        end
        wait(2)
        untagHumanoid(humanoid)
        connection:disconnect()
    else
        damage = damage / 2
        if damage < 2 then
            connection:disconnect()
            ball.Parent = nil
        end
    end

    if math.random(1,3) == 2 then
        explosion = Instance.new("Explosion")
        explosion.BlastRadius = 9
        explosion.BlastPressure = 1000000 -- these are really wussy units
        explosion.Position = script.Parent.Position
        explosion.Parent = game.Workspace
        connection:disconnect()
        ball.Parent = nil
    end

end

function tagHumanoid(humanoid)
    -- todo: make tag expire
    local tag = ball:findFirstChild("creator")
    if tag ~= nil then
        local new_tag = tag:clone()
        new_tag.Parent = humanoid
    end
end


function untagHumanoid(humanoid)
    if humanoid ~= nil then
        local tag = humanoid:findFirstChild("creator")
        if tag ~= nil then
            tag.Parent = nil
        end
    end
end

connection = ball.Touched:connect(onTouched)

r = game:service("RunService")
t, s = r.Stepped:wait()
d = t + 5.0 - s
while t < d do
    t = r.Stepped:wait()
end

ball.Parent = nil