I'm using an iMac with macOS Mojave, ver. 10.14.3. When I try to open Painter 2020 it gives me an error message that it has unexpectedly quit. I had the same problem with Painter 2019.
I'm using Mojave as well, never seen this issue so I wonder what's causing it. Are you using an imported workspace? Did you modify the brush files in any way?
Does the error message hint at anything going on?
It's crashing when I open it, so I can change the workspace or do anything else. No the error message doesn't suggest anything. I had the same problem with Painter 2019.