To bulk load into indexed tables, choose non-parallel mode and disable the Enable Parallel Mode option.
Note that when you disable parallel mode, you cannot load multiple target instances, partitions, or sessions into the same table.
To bulk load in parallel mode, you must drop indexes and constraints in the target tables before running a bulk load session. After the session completes, you can rebuild them. If you use bulk loading with the session on a regular basis, use pre- and post-session SQL to drop and rebuild indexes and key constraints.