TaskPaper 3.5 is setup to recognize Mac App Store licenses, but the latest TaskPaper release in the Mac App Store doesn’t yet copy it’s license to a location that the preview can find. Until version 3.5 is released in the Mac App Store you’ll need to copy the license manually to have it recognized:
Copy this file from the Mac App Store version of TaskPaper:
To this location:
And then the 3.5 Preview should run without asking you to buy a license.