Reason:
if you enter the date directly without specifying the date format, it will cause the inserted time format to be inconsistent with the existing time format in the database, resulting in an error.
:

CREATE TABLE STU_CON
(
No. CHAR(4),
Name CHAR(9),
Sex CHAR(3),
DATE,
Home address VARCHAR2(50),
CONSTRAINT PK_SID PRIMARY KEY (student number),
CONSTRAINT UK_NAME UNIQUE,
CONSTRAINT CK_BDAY CHECK(Date of Birth>'1988-01-01')
)

So you have to specify the date format, and if you can use TO_DATE, you have to declare the date format as well.
for example:
to_date(‘ 1988-01-01 ‘) this would dig its own grave, so it should be written:
to_date(‘ 1988-01-01 ‘, ‘yyyy-mm-dd’)
to_date(‘ 1988-01-01 ‘, ‘yyyy-mm-dd’)