"$" grabs a node, but will return null if it is not present. What you could try is drag and drop the Mob timer onto the code- yes, you can do that- and see what pops up. It could be something as simple as misspelling something.
This might be a lot to take in- I remember being a beginner coder- but there's another way of doing this. I actually prefer using the method "get_node" over #, as it makes more sense in my head.
The way to do this is to, instead of using $, type get_node(), and then in the parentheses, drag and drop the node right in. This avoids any issues with null instances. B)
Not that I'm saying you are a beginner at coding- beginning in Godot is tricky. I remember reading Your First 2D Game and redoing it more times than I want to admit.
