This error is usually related specifically to the username you are attempting to use, but it can occasionally be cause by something else.
Do you have a portal partner?
If you do, it is possible that your partner status is in "error" - this can cause the error you are seeing. This can be checked by any of Verizon's tech support agents.
Also, because I have been unable to find detailed information posted anywhere, here is the information I have gathered regarding all of the fields:
1. The username must be between 6-32 alphanumeric characters, including periods and underscores. The username cannot begin or end with special characters. The username may contain up to two special characters. Special characters may NOT be next to each other or separated by only one character. Usernames may not start with a number.
2. First and Last name must be filled in. (Seems obvious, but I'm trying to cover everything.)
3. DOB must show over 18 for successful sub account creation.
4. Must choose gender.
5. Zipcode must be the same as primary account.
6. Password must be between 6 to 16 characters long, is case sensitive and must have at least 1 number or letter. Passwords must NOT "look" like: your first or last name (as Verizon has it in their system) or the username or vzuid. Passwords can NOT be simple (password1, abc123, etc.) Password should be different from the Primary account's password.
7. You must choose and answer a Secret Question. The answer to the secret question must be at least 5 characters. The answer to the secret question must be different from the password.
Hope this helps! If following those doesn't fix it, call tech support. They have access to see if there is anything wrong with your account from the back end.
Verizon provides an easy to use tool - In Home Agent - to do this for you seamlessly. You can download the tool from http://www.verizon.com/connect.
Once downloaded and installed, just go to the Internet section, then "Email Help" and then "Create an Additional Account". Hopefully this helps.