That is not how node paths work.
get_node() takes the path in the scene tree, not the file path, which is what
res:// refers to.
When the game is running, there are no different "scenes" - that was just how you organized your different objects. When you instance a scene, all of its nodes are created and you add them to the scene tree.
Finding the path from one node to another in the scene tree works exactly like file folders do in your operating system.
get_node("A") gets a child of the current node called "A".
get_node("../B") goes up one level and gets a child of that node called "B" (ie a sibling node).
get_node("/root/C") gets the node "C" that's at the top of the tree.
You should know where in the tree you've added things, but when the game is running, you can also click the "Remote" option in the scene tree to see the live node structure.
This is a very common question and I've written up a more detailed explanation here: