ORA-03297: file contains used data beyond requested RESIZE value

This is because the high water mark on the tablespace extends beyond 12 GB on the tablespace. If you were running Oracle 10g and had Automatic Storage Management on the tablespace and row movement enabled on the tables, you could issue an Alter Table Shrink. With Oracle 9i, unfortunately, it is not as safe and easy. The only options you have with Oracle 9i are to drop the table, resize the tablespace datafile, and then re-create the table; or drop the indexes, move the table to a different tablespace (if one is available with enough space) using Alter Table Move, resize the tablespace, and then move it back and re-create the indexes.