Hi Sapna,
If you go to System and then Workflow and then Environment Locks, you should get an overview of the currently existing Env.locks. In an ideal world the overview will be empty. Provided you have access, you can also delete obsolete locks here.
My guess is that a previous process run did not release the lock and subsequent runs can therefor not create a new lock. If this is the case, you should review the process to verify that if a lock is created, it will always be removed later on in the same process - also when things go bad during the process. In general I would say that locks should be left in place a shortly as possible. So, place a lock, do the thing requiring that lock, and immediately after that remove the lock again.
Happy coding!
---------------
Paul
Sweden
Happy coding!
Paul, Sweden
(By all means, do not mark this as the best answer!)