You need to create the server login "mark" before creating a database user "mark"
e.g. For Windows Authentication
CREATE LOGIN [mark] FROM WINDOWS WITH DEFAULT_DATABASE=[master]
e.g. Or For SQL Server Authentication
CREATE LOGIN [mark] WITH PASSWORD=N'YourPassword', DEFAULT_DATABASE=[master], CHECK_EXPIRATION=OFF, CHECK_POLICY=OFF
Then once the login is created the following should work
CREATE USER [mark] FOR LOGIN [mark]
By the way to get a list of server logins this is the query that management studio uses
SELECT
log.name AS [Name],
CASE WHEN N'U' = log.type THEN 0 WHEN N'G' = log.type THEN 1 WHEN N'S' = log.type THEN 2 WHEN N'C' = log.type THEN 3 WHEN N'K' = log.type THEN 4 END AS [LoginType],
log.is_disabled AS [IsDisabled],
log.create_date AS [CreateDate]
FROM
sys.server_principals AS log
WHERE
(log.type in ('U', 'G', 'S', 'C', 'K') AND log.principal_id not between 101 and 255 AND log.name <> N'##MS_AgentSigningCertificate##')
ORDER BY
[Name] ASC