SQL Server Error while creating User
The error message is stating that SQL Server cannot locate a login named Hiru, or you do not have the privilege required to create a user.

The SQL Server security model consists1 of a server-level login that provides access at the instance level.  Logins are then associated to a user inside each database.  The user typically has the same name as the login, although that is not required.

Creating a login and user is easy to do with T-SQL, as in:

USE master;
CREATE LOGIN [Hiru] WITH PASSWORD = 'pwd';
USE mydb;
CREATE USER [Hiru] FOR LOGIN [Hiru];


Using a T-SQL script provides more direct control over the process, and allows better troubleshooting.



1 - unless you're using the contained database security model