When you create partitions, you should always have an other partition. So any row that does not fit into a given partition will go there. You can then look to see which rows are showing up there and try to figure out why.

Speaking of why, for anyone else to know why you are getting the ORA-14401 error you need to provide the ddl for creating the table and an example of an insert that does not work.

Worst case scenario, hash partitioning usually works well,
and every non null/empty value will probably hash to something useful.