This is because Cubase Pro requires the USB Elicenser.
Did you select the checkbox to buy a USB elc when you bought the program? You can see whether you did in your emailed receipt. In that same email is a link to the store, which is a 3rd party where you created an account, which you can log into.
You might be interested in this: https://helpcenter.steinberg.de/hc/en-us/articles/4408738020498-Free-Grace-Period-Update-to-Cubase-12
