I have been working in Pixelmator Pro more of late and found I was unable to get the eraser side of my old Wacom Intuos 3 Pen to work as an eraser. It’s been quite some time since I last used my tablet so after some searching online and finding that indeed it should work I realized that I plugged in my tablet after Pixelmator was running. So I closed the software, reopened it and it started working again as expected. Hooray!
So be sure to have your Pen Tablet connected to your Mac before you start Pixelmator or you may find that some functions will not work. The more you know… Hope this is of use to others out there that had the same problem. Like most computer issues, when it fails: restart (the computer or app) to see if it resolves the problem.