When you get an error like: How to fix ORA-01654 unable to extend index in tablespace, then this article is for you!.
The problem in Oracle is that when you delete a record, Oracle will leave it blank. That’s a real waste of space when you delete thousands of records. So deleting records will not help to solve this problem.
Here is an example:
Adding 6 records:  After deleting records 2 and 3:  After adding value 7 and 8: 
Fortunately, there is a solution to fix this. You can shrink your tables. By shrinking your tables, Oracle will remove all blank records. To shrink a table, use the following SQL statement.
alter table <mytable> enable row movement; alter table <mytable> shrink space; alter table <mytable> disable row movement; analyze table <mytable> compute statistics; commit;
If that doesn’t work for you, then you have not enough disk space left. Adding some disk space will solve your problem too.
Hopefully your problem is solved now. Enjoy!