Why am i getting an error on my SQL code?

this is my code:

CREATE TABLE dougl_class (

Classcode char (4),

Class_description varchar (20) UNIQUE NOT NULL,

);

This is my error message:

Error starting at line : 28 in command -

CREATE TABLE dougl_class(

Classcode char(4), Class_description varchar(20) UNIQUE NOT NULL,

)

Error at Command Line : 31 Column : 1

Error report -

SQL Error: ORA-00904: : invalid identifier

00904. 00000 -

"%s: invalid identifier" *Cause:

*Action: