In Nero Start, have you logged in with your Nero Account correctly? If you bought Nero product via Nero.com, you need to log in with the email address which you entered during purchase. If you log in with other email, you could not unlock your Nero product. See how to log in in below article: http://support.nero.com/en/support/solutions/articles/44001798936
If you have already log in correctly with your Nero Account, but Nero is still a Trial. In this case, the reason is what you bought is Nero Platinum 2021 permanent license ('pay once, forever use' version), but what you installed is 1-year license subscription version. Please follow below steps:
2. Restart your PC as soon as uninstallation finished.
3. Download Nero 2021 Unlimited to your PC: https://www.nero.com/download.php?id=npla2021_full_installer