You can always buy just the latest Cubase version. Which is Cubase 13 at this moment. With Cubase 13 license, you can start also Cubase 12.
If everything went right, you should see Cubase 11 (Not upgradable) license on your USB-eLicenser and Cubase 13 liense in the Steinberg Activation Manager.
If you have already used a Cubase 11 licence to upgrade to Cubase 12, then the Cubase 11 licence cannot be upgraded again. You must buy a Cubase 12 to 13 upgrade, and apply that to the existing Cubase 12 licence.