That just means you can’t use that specific Cubase 11 License to upgrade to Cubase 12 because it has already been used for that purpose.
- You 've already upgraded the license to C12.
- You have not yet activated your computer to use that license.
- Even though you have not activated C12 on your computer, it will still run for a limited time unactivated
- The notice you got was telling this limited time is running out