Oracle 23c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

select * from nls_session_parameters where parameter = 'NLS_DATE_FORMAT'

PARAMETER
VALUE

NLS_DATE_FORMAT
DD-MON-RR

select TO_DATE ('20-JUN-2010') from dual

TO_DATE('20-JUN-2010')

20-JUN-10

-- to get a different error with the same query
alter session set nls_date_language = 'FRENCH'
select TO_DATE ('20-JUN-2010') from dual
ORA-01843: An invalid month was specified.

-- to get your error with the same query
alter session set nls_date_format = 'YYYY-MM-DD'
select TO_DATE ('20-JUN-2010') from dual
ORA-01858: A non-numeric character was found instead of a numeric character.

-- fix using explicit conversion
select TO_DATE ('20-JUN-2010', 'DD-MON-YYYY', 'NLS_DATE_LANGUAGE=ENGLISH') from dual

TO_DATE('20-JUN-2010','DD-MON-YYYY','NLS_DATE_LANGUAGE=ENGLISH')

2010-06-20

-- fix using a date literal
select DATE '2010-06-20' from dual

DATE'2010-06-20'

2010-06-20