PowerPoint expects you first to make a selection, and then do something such as clicking a button to perform an action for the selected slide objects. If you cannot select an object, then you cannot modify it at all. Although this tutorial explains how you can select shapes on a slide, the process works the same way for any other slide object.
To learn more, choose your version of PowerPoint. If we do not have a tutorial for your version of PowerPoint, explore the version closest to the one you use.