TaskPaper Doesn't Open in OS Sierra

You can’t use this version of the application “TaskPaper.app” with this version of macOS.

I think that this might be a problem with an installation in your computer. Did you buy this one online or in the AppStore? I would probably just uninstall the app (google, “free Mac uninstaller”, App Cleaner is the most popular) and install again.

I think you must have TaskPaper 2? Unfortunately TaskPaper 2 won’t work on macOS 10.12. A technology that it relied on (garbage collection memory management) was removed from 10.12… Oddly TaskPaper 1 does still work, since it didn’t rely on that technology.

You’ll need to upgrade to TaskPaper 3 (50% off for existing customers) for 10.12 support.