When you see below dialog, usually this is because there is an invalid serial number in your system. For example, if you have refund any product, the serial number is invalid. Or some trial serial number is not valid anymore. 

Please click the Remove button in the dialog to remove the invalid serial number directly. Then Nero applications will be able to run again.

Before removing, you could run Nero Control Center and see the serial number with state is 'Non-Licensed Use'. You could  right click on this serial and remove it via Nero Control Center.