What is unable to cast value to function?

workspace.Button.ProximityPrompt.Triggered:Connect(function()
	game.Players.LocalPlayer.PlayerGui.ConversationGui.Base:TweenPosition(
		UDim2.new(0.5, 0, 0.742, 0),
		"Out",
		"Sine",
		0,5,
		false
	)
	wait(2)
	game.Players.LocalPlayer.PlayerGui.ConversationGui.Base:TweenPosition(
		UDim2.new(0.5, 0, 1.2, 0),
		"Out",
		"Sine",
		0,5,
		false
	)
end)

Hi, this is my template script for future plans, But the current error im getting is ''Unable to cast value to function'' In line 2.

What does it mean and how do I get rid of it?