This message appears when you have entered a supplier/customer account manually and have not checked the flag indicating that it is a registered account.
To add accounts to the address book, you need to use the "Update all" button above the address list. This function downloads all registered accounts for individual contacts and checks the account registration flag.
If you do not wish to perform registered account validation, this can be disabled in the company settings, under the Modules - Address Book tab, by unchecking the "Validate companies and accounts when creating a document" field.