Is there any word on when the new Mac operating system will coordinate so that it can work with Corel Painter 12?
As it is now many are having problems installing Mountain Lion and getting an error 20 when they try to use 'Painter.'...
There is an update here http://www.corel.com/corel/pages/index.jsp?pgid=800161&ppid=2900002 which is supposed to give compatibility with Mountain Lion.
You're not alone in having the problem; many people report the same thing. There are indications that the problem may in some cases be triggered by permissions problems. See Serge's post in this thread:
If that doesn't help, then you'll probably need to take this up with Corel support.