What are the values of the Timer's properties?
https://docs.godotengine.org/en/4.7/classes/class_timer.html#properties
If you're not sure, you can print them:
var timer = get_node("Timer")
print_debug("timer.autostart=", timer.autostart)
# Print the other properties.
...
That will also show whether the Sprite2D's _ready function, which connects the timeout signal, is executed.
And what's your Godot version? (The latest version is 4.7.2-stable.)