In recent update, for Nero 2020 and Nero 2021 lifetime license e.g. Nero Platinum or Standalone products, it is required to log in your Nero Account at least once per 7 days in order to get the latest product status of your Nero Account. 

If you see below dialog, usually it is caused by your Nero Account is not logged in.

Please run Nero Start, click on the top right icon and log in with your Nero Account. Then you could start Nero applications.