local events = game:GetService("ReplicatedStorage").Events
local StarterGui = game:GetService("StarterGui") --also use :GetService

events.Added.OnClientEvent:Connect(function(plr)
    if plr.UserId == 89915439 then
        StarterGUI:SetCore("ChatMakeSystemMessage", {
        Text = plr.Name.. " has come to give you a Cat Die!";
        Color = Color3.fromRGB(0,255,255);
        Font = Enum.Font.SourceSansBold;
        FontSize = Enum.FontSize.Size28;
        })
end) 

events.Removing.OnClientEvent:Connect(function(plr)
    if plr.UserId = 89915439 then
        StarterGUI:SetCore("ChatMakeSystemMessage", {
        Text = plr.Name.. " has left the server. Bye Bye Kitty!";
        Color = Color3.fromRGB(0,255,255);
        Font = Enum.Font.SourceSansBold;
        FontSize = Enum.FontSize.Size28;
        })
end)

This LocalScript is inside StarterPlayerScripts, and the only error is that it expects an identifier for ")" at line 12

Anonymous