Autodesk no longer sells Autodesk SketchBook Pro.
Autodesk SketchBook is FREE. It contains the full toolset. To access it, all you need is an Autodesk account. You can try it for 7 days. Download the trial and see what you think.
If you downloaded SketchBook Pro or the mobile version and it is no longer listed on the App stores, check out Finding older versions of Autodesk SketchBook.