If you see this message when you try to create or change your account settings, it's possible that you've already created a myPhoneDesktop Account with that username or this account is used by someone else.
Account names are unique and it is recommended to use your email as unique account name or if you have multiple iDevices then you can use the following schema: "iDevice type+email" when creating an account.
For example: email@example.com and/or firstname.lastname@example.org
If you use this pattern then you are guaranteed to have a unique account name.
How-To Change Account Settings?
How-To add Multiple Accounts?