You are not a member of this portal tier
This may occur when you have been logged into one portal in your browser and try and login to another portal without having logged out (e.g. While logged into the Customer portal you open another tab to login to the account portal). Just use the logout button and login again.
It can also occur if you use customer credentials for the Account Portal and vice-versa. Ensure you use customer credentials for the customer portal and account credentials for the account portal.