You are trying to add a new user to your account, but you are getting a duplicate user error (UniqueConstraintError):
"This is a duplicate of an existing user and cannot be saved."
However, the name of your new user is not in use in your account.
Under Manage Users set Active filter to <All> to see all users
If you can only see users for one company contact your Account Admin, the missing user likely has a role above you
Usernames must be unique among all accounts, so we recommend using an email address as their username.
- For example, the username "Mary" may be unique at your company, but there is another user with that username in our system, so you cannot add a user "Mary" or "mary" to the service.
If the email address is not accepted as a unique username add a number,