Where error in my code?

ORA-00922: missing or invalid option
ORA-06512: at "SYS.WWV_DBMS_SQL_APEX_210100", line 673
ORA-06512: at "SYS.DBMS_SYS_SQL", line 1658
ORA-06512: at "SYS.WWV_DBMS_SQL_APEX_210100", line 659
ORA-06512: at "APEX_210100.WWV_FLOW_DYNAMIC_EXEC", line 1855


4. Semester VARCHAR2(15) NOT NULL,     
5. Group_Class VARCHAR2 (5) NOT NULL,
6. Total_Offer NUMBER(10));

CREATE TABLE OFFERED COURSES
(Offered_Courses_ID NUMBER(15) CONSTRAINT Offered_Courses_ID_pk PRIMARY KEY,
Course_Name VARCHAR2(50) NOT NULL,     
Semester VARCHAR2(15) NOT NULL,     
Group_Class VARCHAR2 (5) NOT NULL,
Total_Offer NUMBER(20) NOT NULL);