Because you are not using correct node paths. I'm assuming this script is on
get_node("fighter"), which means "Get a child of this node named 'fighter'".
fighter doesn't have a child named "fighter", so you get a "null instance".
In addition, you're also trying to use
$fighter2, which also isn't a child of the scripted node. The path from
fighter2 would be
"../../fighter2/fighter2". I don't know why each of your Sprites has a Node2D parent, but that's another issue.
If you're having trouble understanding node paths, you might find this helpful: