Oracle 12C, cannot drop NOT NULL constraint on a DEFAULT ON NULL column

In windows7 I Install oracle 12 C

Then I connect to database as sys user and create new user so:

CREATE USER C##USER
IDENTIFIED BY user_pass;

GRANT ALL PRIVILEGES TO C##USER;

Then I connect to database as C##USER and create table

CREATE TABLE autos (
    id INTEGER GENERATED BY DEFAULT ON NULL AS IDENTITY UNIQUE,
    OWNER_NAME NVARCHAR2(50)
);

and when I am trying insert rows into table, I got problems:

INSERT INTO autos
(owner_name)
VALUES
('Tommy');

On this query, oracle returns: ORA-30667: cannot drop NOT NULL constraint on a DEFAULT ON NULL column

When I connect as sys user, this error not occurred, this error happens only when I connect to database as new created user (in this case this is user: C##USER)

Can you tell me please what is wrong here?

P.S. This error returned both, in SQL Developer and in SQLPlus also.

Thanks