If your user account is integrated as a client account into a Corporate System, you will see this message upon accessing the Upgrade page:
Your account in this state cannot access the regular Upgrade Page. Therefore you have to contact the administrator of the Corporate System (your CPA, most of the time) your account is integrated in and ask them to remove your account temporarily. Afterwards, please purchase the needed premium membership on the same way as you did the first time.
After the purchase, please contact the administrator to re-add your account to the Corporate System.