I have this script to say "Hello there PLAYERNAME." but can't get it to work?

textButton = script.Parent textButton.MouseButton1Down:connect(function() print "Hello there"..username.."." end)

Simple, you just need to get the player's name.

Since this is a text button (A GuiObject), it's best if you use a LocalScript.

IF you use a LocalScript, you can just get the username like so:

local username = game.Players.LocalPlayer.Name

LocalPlayer is a 'local property' of the Players service. It can ONLY be accessed by LocalScripts and it returns the Player object the LocalScript is running on.

Name is a property of every instance in the Roblox API. It is, simply, the name of the Instance. Player objects are always named after their player's username, so it can be used to get a player's username. Simple.

~coo1o