No problems shows in the editor, i can't seem to be able to fix this.
local function leftClick() game:GetService("StarterGui"):SetCore("ChatMakeSystemMessage",{ Text = ("Angry Munci has Spawned!"); Color = Color3.new(255,255,255); Font = Enum.Font.Creepster; FontSize = Enum.FontSize.Size18; }) end script.Parent.MouseButton1Click:Connect(leftClick)
I don't see anything wrong with your code, make sure to run the script in LocalScript
and I think it should work for you. :)