A you are working on a busy database than you will often see this error.
It means that your session is trying to update an object that locked by another session. More specifically, your session has asked for a lock on the same object, but has specified the ‘nowait’ clause. The nowait clause, upon finding something locked, returns an error, rather than waiting for the lock to be released.
- Run your script some other time, when database becomes idle or the object you want to update is lock free.
- Kill the session that has the lock on the object.
For killing locked sessions following is the link to post that explain it in detail: