You can't use
card.get_node("Sword") before you've added the card to the scene tree.
First, you've made
card an instance of the CardDB scene (which contains several nodes). These nodes must be added to the tree if you want to access them.
The next problem is, you're trying to use
add_child() on one of the subnodes of
Presumably, you have a number of cards as children of CardDB. Try this: add CardDB to the tree and then use
hide() to show the particular card you want.
It's hard to give you any more specific advice without knowing more about your setup. I would recommend reviewing this doc which covers the basics of instancing scenes.
PS - please format your code by placing four spaces in front of it when you post a question, so that it appears
and the underscores and indentation aren't stripped away.