why am i getting an error in my sql code?

my code:

ALTER TABLE student_class ADD CONSTRAINT student_class_classcode_fk FOREIGN KEY (classcode);

mye error:

Error starting at line : 53 in command - ALTER TABLE student_class ADD CONSTRAINT student_class_classcode_fk FOREIGN KEY(classcode)

Error report - SQL Error: ORA-00905: missing keyword 00905. 00000 - "missing keyword"

*Cause:

*Action: