Before version 8, the top edition of Cubase (which is now called Cubase Pro) was just called Cubase. Steinberg only allow you to upgrade to the same or a higher edition of Cubase. What you are trying to do is, in effect, a downgrade from Pro to Artist.
You have bought an upgrade from an old version of Cubase Artist to Cubase 10 Artist - but you do not own Cubase Artist so the activation code will not apply. Check in eLicenser Control Center - if your existing licence is Cubase 6, you will need to buy an upgrade from Cubase 6 to Cubase 10 Pro.