In my script, I was trying to get the humanoid of the player instead of game.Players.LocalPlayers.
--Script
--Varibles
local plr = game.Players.LocalPlayer
-- Functions
script.Parent.MouseButton1Click:connect(function(hit)
plr.PlayerGui.NotifyGui.Enabled = true
wait(.5)
end)
How do I get it to grab the Humanoid from the workspace?
UI Properties w/ UI Screen

2 comments
You should use local scripts when dealing with GUIs. You'll encounter errors if you don't, as MouseButton1Click (that's part of a GUI Element) won't work with a server script.
'LocalPlayer' won't work either. This property is only visible to LocalScripts.