Do you have a login named ab12? You can look under the security section at server level, or use this command:
SELECT * FROM sys.server_principals WHERE type_desc = 'SQL_LOGIN' and name = 'ab12';
If there is, and you are still getting the error, you may not have the required permissions to create a user.
If there is no login, you have to create a login before you can create a user.
You can also create users without login, but you will have to specify explicitly so, but more likely than not, that is not what you are trying to do/need. If you do want to create a user without login, see here: http://msdn.microsoft.com/en-us/library/ms173463.aspx