Not all arguments converted during string formatting in operator %

"ready" is a built-in signal for the Node class: https://docs.godotengine.org/en/stable/classes/class_node.html#signals

Using it for another purpose isn't a good idea. I would change that to a custom signal. I know you tried that before, but the Godot signal feature works. If it doesn't appear to be working, it's probably some other issue with your code.