Changing gui text with remote events?

The Problem

local client = game:GetService("Players").LocalPlayer

^ You'd have to do the GetService() separately. You can't get the child after you've called the instance on the same line, which is GetService(), so if you still want to GetService() of the service 'Players' look below.

Replace that with...

local players = game:GetService("Players")
local client = players.LocalPlayer

^ This calls the instance AND gets the LocalPlayer, the below line of code is also is another way to do this. But the above code is more efficient.

For now I'll use this..

local client = game.Players.LocalPlayer

Final Product:

--// Server Script
local event = Instance.new("RemoteEvent")
event.Parent = game.Workspace
event.Name = "MyServerEvent"
game.Workspace.MyServerEvent:FireAllClients()


--// Local Script
local client = game.Players.LocalPlayer
local playergui = client.PlayerGui
local event = game.Workspace:FindFirstChild("MyServerEvent")

event.OnClientEvent:connect(function()
    playergui.ScreenGui.TextLabel.Text = "testing"
end)

Hope it works! :)