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:
/Applications/TaskPaper.app/Contents/_MASReceipt/receipt
To this location:
/Users/<username>/Library/Application\ Support/TaskPaper/Receipts/receipt
And then the 3.5 Preview should run without asking you to buy a license.