I had some problems yesterday activating a license for my copy of Autodesk Sketchbook Pro 2011 so I contacted their technical support.
Autodesk have just decided in the last few days to make Sketchbook a free product.
For Windows desktop apps and Mac you can download it from here: https://www.sketchbook.com/
If you are on Windows 10 (and can face using their app store) you can get it here: https://www.microsof...tchbook/9nblggh4vzw5
There are also Android and iOS versions (though they seem to have some sort of subscription - I guess to to do with cloud sahring between all your devices).