Unable to upgrade Cubase Artist 12 to Cubase Pro 12

Whenever I try to upgrade my Cubase Artist 12 into Cubase Pro 12, an error message comes up saying “Your e-mail address is already used in the system. Please choose another e-mail address or use your old account.”

This is not an official Steinberg support forum.
Please go to your MySteinberg account and issue a support ticket there.

Presumably you have an account already, so log into it instead of creating a new one.