Is the Label a child of the Player Node? Did you try using the "% Access as Scene Unique Name" option, i.e.
get_node("%Label").text = "%s" % health
Can you share a screenshot of you scene tree?
I tested it real quick and it seems to work for me without issues:
I then moved the Label furhter up the tree, but used the "Access as Scene Unique Name" option and it still worked with the code I wrote above: