Am I doing something wrong (Masks)?

Good evening, I am trying to scroll some text and I need it to stay within a rectangle boundary. I am learning how to use masks. Isn’t it as simple as doing the following:

-On my first layer, draw a piece of text outside the boundary on the left side.
-Create keyframe on frame 20 and place text on outside the boundary on the right side and create motion tween
-Create new layer above current layer (call it mask) and draw a rectangle shape for the actual boundary
-Right click mask layer and click ‘mask’

This should allow me to scroll my text horizontally from outside the left of the boundary to the outside of the right side? Or am I doing something wrong??

I need help. My .fla file is attached. Thanks!

EDIT- if you wanna attach a new fla file with it actually working, I would really appreciate it!