Setuser failed because of one of the following reasons: the database principal 'USER-XXX' does not exist,

Hi,
I am in the process of integrating ReqPro (a requirements management tool from IBM Rational) with SQL
Server Express Edition 2005,SP3 in the lab setup.
As a part of process I have created a database named RequisitePro, a user id and a password which is
mapped to the Database. (The Login has been created unchecking the option enforce password rules). This seems to be fine as our AD password rules might be overriding SQL.
Now the Database and user (having owner and sysadmin permission) has been setup.verified logging in with
SQL Server Authentication (mixed mode enabled).
when executed the below query it is throws a error. setuser go If exists (select * from sysusers where uid = user_id('ReqPro'))     setuser 'ReqPro' 
Error Message:
Msg 15157, Level 16, State 1, Line 3 Setuser failed because of one of the following reasons: the database principal 'ReqPro ...