Ok.
That all makes sense now.
That error actually does mean something - you won't be able to see the scaling effect when it happens, but otherwise should be harmless.
How you want to set this up is -
Button (tk2dUIItem, tk2dUISound, tk2dUIUpDownButton)
GraphicRoot - tk2dUITweenItem
SlicedSprite_Up + text (Remove UITweenItem if copied from basic sprite)
SlicedSprite_Down + text (Remove UITweenItem if copied from basic sprite)
or
Button (tk2dUIItem, tk2dUISound, tk2dUIUpDownButton)
GraphicRoot - tk2dUITweenItem
SlicedSprite_Up
SlicedSprite_Down
Text - use this method if text doesn't need to change between up and down
Since its swapping between the up and down states, the scale tween should be performed by something that does it on both of them and DOESN'T get disabled. I.e. the Root object above. Example attached.