"Unable to cast string to int64" even though the argument is an int?

I tried tonumber() an already number. Didn't work

Server Script

repeat wait() until game.Workspace

local TeleportService = game:GetService("TeleportService")

local Remote = script.Parent.Event

Remote.OnServerEvent:Connect(function(SentFrom, Setting, ExtraData)
    if string.lower(Setting) == "teleport" then
        local finished, err = pcall(function()
            local Code = TeleportService:ReserveServer(ExtraData)
            TeleportService:TeleportToPrivateServer(4805466213, Code, {SentFrom})
        end)

        if not finished then
            warn(err)
        end
    end
end)

Local Script

function GenCode(Len, Al, Num, Special, Space)

    local Len_Code = Len

    local Alphabet = Al
    local Numbers = Num
    local SpecialCharacters = Special
    local Space = Space

    local alphabet = 'AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz'
    local numbers = '1234567890'
    local specialcharacters = '{}!@#$%^&*()_+-=[]\\/.,*+'
    local space = ' '
    local output = ''

    local function ReturnUsable()
        local returntable = {}
        if Alphabet == true then
            table.insert(returntable,1,alphabet)
        end
        if Numbers == true then
            table.insert(returntable,1,numbers)
        end
        if SpecialCharacters == true then
            table.insert(returntable,1,specialcharacters)
        end
        if Space == true then
            table.insert(returntable,1,space)
        end
        return returntable
    end

    for i = 1, Len_Code do
        local tables = ReturnUsable()
        local CurrentType = tables[math.random(1,#tables)]
        local a = math.random(1,string.len(CurrentType))
        output = output..string.sub(CurrentType,a,a)
    end

    return output
end

function CreateCode()
    return GenCode(6, true, true, false, false)
end

function CreateGame()
    local Code = CreateCode()
    Remote:FireServer("Teleport", Code) 
end

CreateBtn.MouseButton1Click:Connect(CreateGame)