In our new Nero 2020(Nero Platinum Suite), we use account system to manage your products and serial numbers. 

Your Nero Account consists of the email address you used when you first signed up or purchased a Nero product. It helps you to use your Nero applications and services securely and reliably.

With Nero Account, you can store all your Nero products in a single account. Once you login with your account, your serial numbers will be added into your system automatically  and shown as "-" in Nero Control Center as designed.  So you don't have to enter your serial number again, neither remember all your serial numbers. 

Sorry for the confusion.