So first off I would say you might want to change the:
PLAYERBOX | On collision with SCROLLY
to:
PLAYERBOX | Is overlapping SCROLLY
I believe that On collision with is only a single event, so you would only get one step out of the lerp expression. The Is overlapping will act like an every tick event as long as you are overlapping the object.
Next I'm wondering if your statement:
...PLAYERBOX platform is moving = | lerp(CanvasToLayer(0...
is a typo or not. Did you really mean to type:
...PLAYERBOX platform is moving = || System | Set Scroll Y to lerp(CanvasToLayer(0...