Unfortunately, this issue occurs when Apple's servers fail to send a receipt file to you Mac after you complete an In-App Purchase. We have notified Apple of this bug, but in the mean time, you can manually fix the issue by following the steps below:
Delete the app from your Mac via the Launchpad app. (This is more effective than dragging it to the Trash can.) To open Launchpad, first click on the Spotlight icon and search for 'Launchpad' and press the Enter key. Within the Launchpad, click and continue to hold on the app icon, then click the little “X” icon that appears. If you are unable to open Launchpad, delete the app from the Applications folder instead.
Log out of the Mac App Store (from the menu bar, choose Store -> Sign Out).
Reboot your Mac.
Log back in to the App Store (Store -> Sign In), then switch to the Purchases tab and click the install button next to the app you want to re-install.
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