ORACLE 12 C, cannot drop NOT NULL constraint on a DEFAULT ON NULL column - oracle

Question

create table autos (
id integer generated by default on null as IDENTITY unique,
owner_name nvarchar2(50)
);
Then I insert in table several rows
insert into autos
(owner_name)
VALUES
('Nick');
insert into autos
(owner_name)
VALUES
('Tommy');
2 rows inserted, then for increase rows count, I run this query
insert into autos
(owner_name)
select owner_name
from autos;
Several query was run successfully, but after this, oracle returns error: ORA-30667: cannot drop NOT NULL constraint on a DEFAULT ON NULL column
Tell please, what is wrong here?
P.S. I use SQL Developer.
UPDATE
If I am trying all above codes in sys database connection, all works fine, but I am create new user (here is code how I create new user)
CREATE USER C##OTO_USER
IDENTIFIED BY oto_user_pass;
GRANT ALL PRIVILEGES TO C##OTO_USER;
Then I create new connection with C##OTO_USER and only in this connection happens above error.
Also, that error happens sometimes, sometimes INSERT query works fine.
And not only INSERT... SELECT, but usually INSERT statement also causes that error.
So, I think this is new user/connection problem, may be above user creating code, not creates complete user?
If trying inserting with SQL*PLUS, also same error happens.

Answer 1

Your problem is that when you use IDENTITY in the column definition, you should take into account its restrictions.
If you want to insert rows as subquery so you encounter with following restriction: CREATE TABLE AS SELECT will not inherit the identity property on a column (source). Same situation happens with your insert, IDENTITY unique doesnt work properly.
insert into autos
(owner_name)
select owner_name
from autos;