ORA-01843: not a valid month

Problem:

When trying to convert the char column (attribute9) to date using the to_date function in SQL Developer, the following error is displayed:

ORA-01843: not a valid month
01843. 00000 -  "not a valid month"
*Cause:   
*Action:

Solution:

Dates were stored in two different formats (mm/dd/yyyy) and (dd-mon-yyyy) in the attribute9 column. Convert all dates to one format (dd-mon-yyyy).