[simbr]

The failure means that no-one else is using the address.
When an username is removed from an account it remains locked to that account for 6 months to prevent someone else registering it in case you want it back. In this case the someone else is you, but the system doesn't know that since you are using a different account. You will have to contact customer or technical support to see if they can unlock or migrate the username for you.