alter table < table_name> modify( <column_name> not null) does not work in parallel

I have done the following (Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production))

Issue is: it never runs in parallel. Other process do however.

example table sched_step

alter table SCHED_STEP parallel (degree 8);

ALTER TABLE SCHED_STEP

 MODIFY (CREATION_DB_ID NUMBER NOT NULL),

     CTRL_DB_ID NUMBER NOT NULL);

this takes about 40 minutes but never goes parallel.