The check constraints checking multiple values should have IN operator
Try:
CREATE TABLE STUDENT (
SNO NUMBER(6),
SNAME VARCHAR2(10) CONSTRAINT SNAME_NN NOT NULL,
COURSE CHAR(10),
FEE NUMBER(10) CONSTRAINT FEE_NN NOT NULL,
DOB DATE CONSTRAINT DOB_NN NOT NULL,
CONSTRAINT SNO_PK PRIMARY KEY(SNO),
CONSTRAINT SNAME_CK CHECK(SNAME=UPPER(SNAME)),
CONSTRAINT COURSE_CK CHECK(COURSE IN('JAVA','ORACLE','.NET')),
CONSTRAINT FEE_CK CHECK(FEE>500)
);