ORA-13624: The task string is executing and cannot be deleted or modified.
Cause : The user attempted to access a task that currently executing.
Action - How to fix it : DBA Scripts :: www.high-oracle.com/scripts
Wait for the task to complete and retry the operation.
Well, the reason for above error is that i have taken the above script from a 11g database and running it on 10g database. 11g has bring some changes in password management. Below code is executed on 11g and user created successfully, which is expected result.