Godot "your first 2D game" : node not found

Godot Version

4.2.1

Question

I wanna try to make “your first 2D game”
I finished my project until Heads up display

before entering the section “removing old creeps”, I faced to the problem.
when I try to play the project, I can find the error such as ‘show_message():Node not found: MessageTimer(relative to~)’

How can I solve this problem.
Thank you so much

this will give the same error

func _ready():
	print($doesnt_exist)

make sure that HUD.tcsn has all of its children in your main.tscn