According to the version 3 release notes it's time for an upgrade to Sierra at least. The Line6 support page has all this information and much much more. Compatible OS: Mac OS X, macOS Sierra, macOS High Sierra, macOS Mojave, macOS Catalina
Add one more name to the list. Mid 2012 13 MacBook Pro. I'm trying to upgrade 2.21 to 2.30. First problem was with the backup because I had the problem where the USB connection was lost. Solved that one by following the instructions and shutting down everything that might want to say hello to to USB audio. When the backup finished and I used the updater app I got an unsuccessful update and I'm stuck in "Enter Update Mode". I really hoped they would have provided some guidance on the nature of the mythical mode and how to get get there from the 3rd di-flippin-mention. Keep you posted.