If you can post the following, then we should be able to help you out:
- some sample geometries from your table
- the statement you used to set the user_sdo_geom_metadata
- the statement you used to create the spatial index
- the full error message you get
There is one annoying thing about oracle in creating spatial indexes. Even if the create index call fails it does not clean up after itself and you have to drop the index before attempting to re-create it.