SetCore: SendNotification has not been registered by the CoreScripts?

local function callback(text)
	if text == "Yes" then
		print("They want extra points!")
	elseif text == "No" then
		print("They don't care...")
	end
end
 
local bindableFunction = Instance.new("BindableFunction")
bindableFunction.OnInvoke = callback
 
game.StarterGui:SetCore("SendNotification", {
	Title = "Welcome to my game!"; -- Required. Has to be a string!
	Text = "Do you want extra points?"; -- Required. Has to be a string!
	Icon = ""; -- Optional, defaults to "" (no icon)
	Duration = 5; -- Optional, defaults to 5 seconds
	Callback = bindableFunction; -- Optional, gets invoked with the text of the button the user pressed
	Button1 = "Yes"; -- Optional, makes a button appear with the given text that, when clicked, fires the Callback if it's given
	Button2 = "No"; -- Optional, makes another button appear with the given text that, when clicked, fires the Callback if it's given
})

Just trying it out, and this is code from their wiki. I'm just trying it out to see if it would be useful to implement in a game.

It just means that all the core functions aren't loaded yet, stick a wait(4) (Maybe less) in there and you'll be good.