Why does my script enable the script when it has a certain xp not working?

You shouldn't disable the script. There's no reason to. Use your if statement to determine if code runs instead.

There are a couple more problems with your setup.. i'll list them off.

• KeyDown is deprecated, and should not be used for new work. Use the UserInputService.InputBegan event.

• You're cloning GreenPlasma outside of the event, so it will only occur once. Clone inside of the event.

• The animation object should be created outside of the event, so it is not being created everytime the event fires.

• Lighting is not meant for storage. Use ReplicatedStorage.

• And finally, connect is deprecated. Use Connect.

local player = game.Players.LocalPlayer
local pcoins = player:WaitForChild("leaderstats"):WaitForChild("Xp")
local char = player.Character or player.CharacterAdded:Wait()
local hum = char:WaitForChild("Humanoid")
local GreenPlasma = game.ReplicatedStorage.GreenPlasma --MOVE THE OBJECT
local uis = game:GetService("UserInputService") 
local toggle = false
local animation = Instance.new("Animation")
animation.AnimationId = "rbxassetid://01507595343"
local c;


uis.InputBegan:Connect(function(i,p)
	--second parameter detects whether you're typing in a textbox
	if not p then
		--Check input
		if i.KeyCode == Enum.KeyCode.Y then
			--Check coins
			if pcoins.Value < 320 then return end
			--toggle the toggle
			toggle = not toggle
			--check the toggle
			if toggle then
				hum.WalkSpeed = 0
				local AtcPlay = hum:LoadAnimation(animation)
				AtcPlay:Play()
				wait(1)
				hum.Walkspeed = 24;
				--CLONE INSIDE EVENT
				local copy = GreenPlasma:Clone()
				copy.Parent = char;
				--assign to variable outside event since you use object in
				--multiple scopes
				c = copy;
			else
				if c then
					c:Destroy();
				end
				hum.WalkSpeed = 16
			end
		end
	end
end)

NOTE: *You aren't changing where the copy of GreenPlasma is located.. I suspect you want to do so.