In the editor window, you can always move sprites around with them mouse in order to set their initial position. This is because you are the programmer, and you are editing the sprites.
However, outside of the editor, you can only interact with the program in ways that are defined by blocks. In the program window, you are the user, and you are not allowed to edit the sprites.
Fortunately, you do not need to construct your own mouse-click code from blocks in Scratch; there is a feature that you can use, called the Draggable Sprite feature. You can turn it on by clicking on the
i at the top-left corner of each cat sprite in the editor window and checking the box labeled, "can drag in player."