While doing what ? you are getting error ? whats the situation?
If you are trying to load the data and do get this error, you can get rid of it by setting up commit parameter say for every 10000 rows or so.
The usual reason for this error is that someone else has an open transaction on this table. For example, they (or possibly you) has that table up on their screen from some application or utility. Something like SQL-Stations maybe where they can update a record and it doesn't automatically commit.
Check the SGA for any open cursors that reference that table.