Function won't call?

You're calling the function CastSpell with no arguments when it has a spell parameter. Inside that function, spell.A1:Play() should error because spell is not defined.

On line 72, the connection to the function CastSpell needs to pass whatever necessary argument needs to be passed to that function.

mouse.Button1Down:connect(function()
    CaseSpell(argument)
end)

Also, the function on line 6 does not have an end.

I didn't look through your code because it wasn't tabbed correctly. Please tab your code in the future so it is easy for people to read.

You were missing an "end". You would have caught that if you had tabbed correctly.

local wand = script.Parent
local stick = wand.Handle
local inc = script.Name
local owner, connect

function SpellAffectAvadaKedavra(hit,spell)
    if (not hit.CanCollide) and (hit.Parent:findFirstChild("Humanoid") == nil) then return end
    local hum = hit.Parent:findFirstChild("Humanoid")
    if (hum) then
        wait(.5)
        spell.A2:Play()
        hum.Health = 0
        if hit.Name == "Handle" then
            if hit.Parent.Parent == game.Workspace then
                local i = Instance.new("BodyPosition")
                i.position = wand.Parent.Torso.Position
                i.Parent = hit
                wait(3)
                i.Parent = nil
                spell:Destroy()
            end
        end
    end
end



function Setup()
    for i,p in pairs(game.Players:children()) do
        if (p.Character == wand.Parent) then
            print("Found owner of wand!")
            if (connect ~= nil) then connect:disconnect() end
            owner = p
        end
    end
end

local s1 = script.Parent.Handle.A1:Clone()
local s2 = script.Parent.Handle.A2:Clone()

function CastSpell(spell)
    if game.Players.LocalPlayer.PlayerGui.Spell.Spell.Text.Value == "Spell : Avada Kedavra" then
        s1.Parent = spell
        s2.Parent = spell
        wand.Activated:wait()
        spell.A1:Play()
        local p = Instance.new("Part")
        p.formFactor = "Symmetric"
        p.Shape = "Ball"
        p.Size = Vector3.new(1,1,1)
        p.TopSurface = "Smooth"
        p.BottomSurface = "Smooth"
        p.Transparency = 0.5
        p.BrickColor = script.AvadaKedavraColor.Value
        local sparx = Instance.new("Sparkles")
        sparx.SparkleColor = script.AvadaKedavraSparksColor.Value
        sparx.Enabled = script.HasSparkles.Value
        sparx.Parent = p
        coroutine.resume(coroutine.create(function()
            wait(10)
            p:Destroy()
        end))
        local bv = Instance.new("BodyVelocity")
        p.Parent = workspace
        p.Name = "MagicSpell"
        p.CFrame = stick.CFrame * CFrame.new(0,1.5,0)
        bv.Parent = p
        bv.velocity = ((owner.Character.Humanoid.TargetPoint - p.Position).unit) * 200
        p.Touched:connect(function(part)
            SpellAffectAvadaKedavra(part,p)
        end)
    end
end

wand.Equipped:connect(Setup)

script.Parent.Equipped:connect(function(mouse)
    mouse.Button1Down:connect(CastSpell)
end)