This script works in studio but not online?

local SetText = script.Parent.Text
local player = game.Players.LocalPlayer
local mouse = player:GetMouse()

function UpdateOldLabels(Parent)
    for i,v in pairs(Parent:GetChildren()) do
        if v.Name:sub(1,4):lower() == "line" then
            local LineNumber = v.Name:sub(5)
            if LineNumber == "12" then
                v:Destroy()
            else
                v.Name = "line"..tostring(tonumber(LineNumber) + 1)
                v.Position = v.Position - UDim2.new(0,0,0,15)
            end
        end
    end
end

--[Chat]--
script.Parent.FocusLost:connect(function(EnterPressed)
if EnterPressed then
			for _,v in ipairs(game:GetService("Players"):GetChildren()) do
			if string.find(script.Parent.Text, "/") == 1 or string.find(script.Parent.Text, " ") == 1 or script.Parent.Text == "" or (script.Parent.Text == " ") then
			script.Parent.Text = SetText
			else
	        UpdateOldLabels(v:WaitForChild("PlayerGui").ServerChat.Frame)
            newchatline = Instance.new("TextLabel",v:WaitForChild("PlayerGui").ServerChat.Frame)
            newchatline.Text = "["..player.Name.."]: "..script.Parent.Text
            newchatline.Size = UDim2.new(1,0,0,18)
            newchatline.Position = UDim2.new(0,0,1,-30)
            newchatline.Font = "SourceSansBold"
            newchatline.TextColor = player.TeamColor
			newchatline.TextStrokeColor3 = Color3.new(0,0,0)
            newchatline.TextStrokeTransparency = 0.7
            newchatline.BackgroundTransparency = 1
            newchatline.BorderSizePixel = 0
            newchatline.FontSize = "Size18"
            newchatline.TextXAlignment = "Left"
            newchatline.TextYAlignment = "Top"
            newchatline.ClipsDescendants = true
            newchatline.Name = "line1"
			script.Parent.Text = SetText
	
        
      	  UpdateOldLabels(game:GetService("StarterGui").ServerChat.Frame)
      	  local clonedchat = newchatline:Clone()
		  clonedchat.Parent = game:GetService("StarterGui").ServerChat.Frame
			end
	end
end
end)

mouse.KeyDown:connect(function(key)
if key == "/" then
script.Parent:CaptureFocus()
end
end)

script.Parent.FocusLost:connect(function()
wait(0.01)
script.Parent.Text = SetText
end)

You need to use a LocalScript, which runs locally.

"LocalPlayer" directs a LocalScript to the Player object that the LocalScript is using, but a server script(Simply called, a "script".) cannot use it.

I may be wrong, but I've checked the script exactly five times, and there seems to be no problem if it's a LocalScript, and you said, "this script", so I'm guessing it's a script.

A better answer may be found here: http://wiki.roblox.com/index.php?title=LocalScript