If player joined who is on whitelist, then kick?

Now, I am going to completely redesign your script, the first being an improvement of your one, and the second being a new (but much more efficient) way to do it.

Redesign #1:

local whitelist = script.Folder:GetChildren()

function WhiteListCheck(p)
    for _,v in pairs (whitelist) do
        if v.Value == p.Name then
            return true
        end
    end
end

game.Players.PlayerAdded:connect(function(p)
    local banned = WhiteListCheck(p)
    if banned == true then
        p:Kick("You are banned from this game")
    end
end)

This code cycles through all of the the children of whitelist, checking to see if their value is the Player's name. If it is, it returns true, and the PlayerAdded function kicks them.

Redesign #2

local whitelist = {
    ["PlayerName"] = true,
    ["PlayerName"] = true
}

game.Players.PlayerAdded:connect(function(p)
    if whitelist[p.Name] then
        p:Kick("You are banned from this game")
    end
end)

This code just checks to see if the player is in the table, and if they are, they get kicked.

local WhiteList = script.Folder:GetChildren()
local Defined = script.Folder.Defined.Value
local Player = game.Players.PlayerAdded
local A = script.Folder("1").Value
local B = script.Folder("2").Value
local C = script.Folder("3").Value
local D = script.Folder("4").Value
local E = script.Folder("5").Value
game.Players.PlayerAdded:connect(function()
    if Player.Name == Defined then -- Try Player.Name
    game.Players(Player):Kick("Go away, you have no friends...") -- lol 
    print("Player Attempted to join!")
    elseif Player.Name == A or B or C or D or E then
    game.Players(Player):Kick("Bai!")
    end
end)

Now i may be totally wrong because im tired asf but, Try it . . .