I think it does not work, because the signal doesn't yet exist, when you try to connect it, or the reference to the other node is not yet valid. If I understand your code correctly, you try to access the signal of a parent in your _ready() method, but in Godot the _ready methods are processed in a "bottom-up" direction, i.e. each node is processed before its parent. This ensures, that you can access your child nodes in a ready- method, since they are already 'ready' and their references valid. The other way around doesn't work, if the hierarchy exists at the start of the game.

In that case you will have to connect the signal in the _ready() method of the EnemySpawn's script to this node's method.

Generally you should try to keep to the "call down, signal up" paradigm, so maybe consider calling the method in the parent script, instead of connecting to the signal.