ORA-01821: date format not recognized

This error message is typically caused by attempting to use a date format that is not recognized by the Oracle database. The most common causes of this error include:

1. Using an incorrect or unrecognized date format in a SQL query.
2. Attempting to insert a date value into a column that is not defined as a date datatype.
3. Using an incorrect or unrecognized date format in a PL/SQL block.
4. Using an incorrect or unrecognized date format in a TO_DATE or TO_CHAR function.

To resolve this error, you should verify that the date format you are using is correct and recognized by the Oracle database. You can do this by checking the Oracle documentation or by consulting with a database administrator or developer. Additionally, you should ensure that any date values you are inserting into the database are properly formatted and match the datatype of the column they are being inserted into.