sql - Cannot find the user 'database_principal', because it does not exist or you do not have permission -

sql - Cannot find the user 'database_principal', because it does not exist or you do not have permission -

i trying create notification service in sql server 2008 r2.

but when running command grant subscribe query notifications database_principal. getting below error :

cannot find user 'database_principal', because not exist or not have permission.

can 1 help me on , iam doing wrong ?

sql sql-server sql-server-2008