Please try the following:
- Make sure you are logged into the same Google Play account which you used to make your previous purchases.
- Check that you have the latest update for the Num2 app installed.
- Ensure you have the latest operating system for your device.
- Try to clear the cache & data of the Play Store (this will restart the app and can help fix issues):
- On your Android device, open the settings >> app Settings.
- Tap Apps & Notifications and then See all apps.
- Scroll down and tap Google Play Store.
- Tap Storage and then Clear Cache.
- Tap Clear data.
- Re-open the Play Store and try your purchase again.
Check Google Support for more info on fixing issues with Google Play services.