That happens often after restoring dumps when everything else is OK. Sangeet suugest that in his blog.
http://letstalkclarity.blogspot.fi/2008/07/6-sql-server-2005-configuration-for.html
The option is to rename the current niku user and do again steps
3. Security > Create Login > niku ( Select SQL Server Login and give niku password )
4. Database > Niku > Security > Users > Create User > user Name : niku ; Login : niku
5. Database > Niku > Security > Schema > Create Schema > Schema Name : niku ; Schema Owner : niku
6. Security > Logins > Niku > Properties > User Mapping > Select Niku database Row > For Niku Database change 'Default Schema' to niku > In Below Pane check the role membership for 'db_owner'.
or create a new user with another name.
Though
EXEC sp_change_users_login 'Auto_Fix', 'niku'
is the simplest (if your user name is niku)
Martti K.