The error says just what the problem is - there's no node named "Sprite" attached to this one.
$Sprite means the same thing as
get_node("Sprite"), which translates to "Get a child of this node named 'Sprite'".
It appears you have this script on your Sprite node, so of course there's no child also called "Sprite". If this script is on the sprite that you want to hide, then you don't need to get any node, and you can just say
visible = false