Clear the cache of your device, the cache of “Google Play Games”, “Google Play Store” and “Oddmar”, then reboot the device. After rebooting, try to buy the game again. Also make sure you are using the latest version of the Google Play Games app.
When game shows you the purchase screen, just select the unlock option again and game will give you "You Already Have This Item" message. After that you can enjoy the whole game.
