Just remove the quotes:
Set position to (BlockMoveable(21).BBoxLeft-2, .....
As for the second part (int(selectedObject & ".BBoxTop")), this will not work. You can't refer to objects in C2 by their string name. Same applies to expressions.
It's hard to tell what are you trying to do from the screenshot. If multiple objects/sprites can be selected, then one possible solution would be to add all such objects to a family. Use SelectedObject variable to store UID of the selected instance.
Then do this:
MyFamily -> Pick by unique ID = SelectedObject
.........HighlightBox Set position to (BlockMoveable(21).BBoxLeft-2, MyFamily.BBoxTop-2)