Error 'ora-00001: unique constraint (constraint_name) violated' when inserting data. why?

This means you're trying to insert or update a record that would result in a duplicate value in a column that must be unique.

Example:

ORA-00001: unique constraint (SYS.PK_USERS) violated

Solution:

1. Ensure you're not inserting a duplicate value.
2. Check the specific constraint (e.g., PK_USERS) to determine the unique column.
3. Modify the data accordingly.