It is because this user does not exist in this server. First of all the user must have a login that exists. You can create one if you are a sysadmin. Then you have to associate the login with the database user, because at this point it is an orphan and SQL Server does not know what to do with it. Here you use "exec sp_change_users_login 'Update_one', 'userid', 'userid'". This will reassociate the user id to the database.