First of all, be sure that your device is linked to the same Google account that was used for the purchase. If you are sure, it looks like there is an issue with the Google Play app on your device: it did not get the list of purchases correctly. Typically, the issue goes away in a few hours. But if you want, you can fix it manually. There are two ways.
Way 1 On the device, open the Preferences app. Go to the “Apps” section, and look for “Google Play”. Now click “Force stop”, then “Clear data”. Done? Ok, now start the Google Play app again. It re-reads the data from the Google server, and the issue has gone. Note: there is also a button named “Clear cache”, it does not help.
Way 2 On the computer, open FBReader Premium page on the Google Play, and click the “Install” button. Select your device from the list, and the app will be installed on the device in a few minutes.