At this time, we are in the process of resolving an issue with SketchBook from the Mac App Store may not install. At this time, please download the version from here:
https://www.sketchbook.com/thankyou
NOTE: SketchBook Pro 2020 & SketchBook (link above) will install and work on Mac OS 10.15 (Catalina).
NOTE: If you have SketchBook (Mac App Store Version) already installed on Mojave and you upgrade to Catalina, SketchBook will function.
David Lau