Follow the steps.

1. Draw a rectangle.
2. Select the whole area of rectangle with Arrow Tool
3. Now convert it to a graphic symbol, goto menu Insert and click
  on Covert to Symbol. And from that popup window, select Graphic option. 

  

  Now your rectangle is a graphic symbol.
4. RightClick on frame number 20 and selcect Insert Keyframe.
5. Now LeftCilck on frame number 20, and drag your rectangle to any other position, put it in the left botton corner.
6. Now rightclick on any frame between frame number 1 and 20, and select
  Create Motion Tween.