There are lots of mistakes in those expressions. Fixed:
X: clamp(Player.X, RectangleFocusArea.BBoxLeft+ViewportWidth("Player")/2, RectangleFocusArea.BBoxRight-ViewportWidth("Player")/2)
Y: clamp(Player.Y, RectangleFocusArea.BBoxTop+ViewportHeight("Player")/2, RectangleFocusArea.BBoxBottom-ViewportHeight("Player")/2)
Also, the size of RectangleFocusArea needs to be larger than the window (viewport) size, otherwise clamp will not work.