You are not allowed to use uppercase letters when you create a new name in Yahoo!. You cannot use an \"_\" at the beginning or end of a name.

With millions of users, many names are already taken, so think unique when registering for a new one. If you want a name similar to the one you first tried, try finding a creative way to alter it, such as adding an \"_\" (underscore) and the current year to the end of it. 

Not all Yahoo! Profiles are listed in the Yahoo! Member Directory. Even if an ID or alias doesn’t come up in your search, it may still exist.