Error #8: Cannot connect to server

Ensure that you have stable internet connection and Play Store app loads its content. Then force stop the main app (Call Recorder) and open it again.

Error #13: The app is not published on the Play Store yet

Clear app data of the Play Store app and Google services, then reboot the device.

Error #103: The app cannot connect to Unlocker app to ask for license check

Open Unlocker app (if you hid icon launcher, then press on "License status" in Settings > About). Then force stop the main app and open it again. Greenify and other similar apps may cause this issue.

Error #104: Unlocker app cannot connect to the Play Store

Fix: Clear app data of the Play Store and Google services, then reboot the device.

Other possible reasons

1. The app didn't check a license yet.
   
   Solution: Just wait several minutes, then close and open settings again.

2. Something wrong with the Play Store.
   
   Solution: Clear app data of the Play Store. Force stop the app. If it won't help, try to uninstall Play Store's updates, then reboot device.

3. Installed LuckyPatcher, Freedom or ROM with modified core.jar.
   
   Solution: Make sure the app has access to the internet. You need to stop Freedom before license check. Force stop the app, then open. Wait a few minutes. If it doesn't help, probably you need to disable LuckyPatcher or/and revert core.jar (reboot after that).

4. No internet connection.
   
   Solution: There's no way to check a license without internet connection. So ensure your device is connected to the internet. You don't block it. Then force stop the app and open it. Wait a few minutes.

5. Greenify.
   
   Solution: If you're using this app, disable it then reboot the device. Now the app should be able to check a license. After successful license check you can enable it back till next update.