You are calling
get_node on something that is not a node!
First, instantiate the object:
var camera = load("res://scripts/Camera2D.gd").new()
Then, you can just call the function:
However, in your case, it seems like you want to do something using a node that is already in the scene that you're using. In that case, you can just do:
camera_node is your camera with the camera script attached and it is a direct child of the node that owns the script from which you want to do the call.