SQL> ed
Wrote file afiedt.buf

  1  create or replace trigger trgr
  2  before insert on myTable
  3  for each row
  4  begin
  5  select mySequence.nextval into :new.ID from dual;
  6* end;
SQL> /
create or replace trigger trgr
                          *
ERROR at line 1:
ORA-04089: cannot create triggers on objects owned by SYS

ORA-04089: cannot create triggers on objects owned by SYS

create or replace trigger trgr
  before insert on myTable

   create table mytable(id number); 
    
   create sequence mySequence  start with 1 increment by 1 ; 

    create or replace trigger trgr
        before insert on myTable
        for each row
    begin
          select
                       mySequence.nextval 
           into        :new.ID 
           from         dual;
        dbms_output.put_line(' sequence vqalue ' || :new.ID);
   end;

   insert into mytable values (12367);
   insert into mytable values (8596); 
   insert into mytable values (989); 

  But the out put 
  
   Select  *  from mytable; 
    
   Id 
   1
   2
   3

SQL> conn user1/user1 @myNSN as sysdba
Connected.
SQL> create table myTable (ID number);

Table created.

SQL> select owner from all_tables where table_name= 'MYTABLE';

OWNER
------------------------------
SYS


SQL> ed
Wrote file afiedt.buf

  1  create or replace trigger trgr
  2  before insert on myTable
  3  for each row
  4  begin
  5  select mySequence.nextval into :new.ID from dual;
  6* end;
SQL> /
create or replace trigger trgr
                          *
ERROR at line 1:
ORA-04089: cannot create triggers on objects owned by SYS

SQL> conn user1/user1 @myNSN
Connected.
SQL> create table tbl (ID number);
create table tbl (ID number)
*
ERROR at line 1:
ORA-01031: insufficient privileges

select default_tablespace from dba_users where username='<your user (in upper case)>';

SQL> create user user2 identified by user2;

User created.

SQL> conn user2/user2 @myNSn as sysdba
Connected.
SQL> grant create any table to user1;

Grant succeeded.

SQL> conn user1/user1 @myNSN
Connected.
SQL> create table tbl(ID number);
create table tbl(ID number)
*
ERROR at line 1:
ORA-01950: no privileges on tablespace 'SYSTEM'

SQL> CONNECT sys/anythingr@myNSN AS SYSDBA
Connected.
SQL> SHOW USER
USER is "SYS"
SQL> CREATE USER userdba IDENTIFIED BY userdba
  2  /

User created.

SQL> GRANT DBA TO userdba
  2  /

Grant succeeded.

SQL> CONNECT userdba/userdba@myNSN
Connected.
SQL> SHOW USER
USER is "USERDBA"
SQL> CREATE USER user1 IDENTIFIED BY user1
  2  DEFAULT TABLESPACE users QUOTA 10M ON users
  3  TEMPORARY TABLESPACE temp
  4  /

User created.

SQL> GRANT CREATE SESSION, CREATE TABLE, CREATE SEQUENCE, CREATE TRIGGER TO user1
  2  /

Grant succeeded.

SQL> CONNECT user1/user1@myNSN
Connected.
SQL> SHOW USER
USER is "USER1"
SQL> CREATE TABLE myTable
  2    (seq  NUMBER,
  3  	id   NUMBER)
  4  /

Table created.

SQL> CREATE SEQUENCE mySequence
  2  /

Sequence created.

SQL> CREATE OR REPLACE TRIGGER myTrigger
  2    BEFORE INSERT ON myTable
  3    FOR EACH ROW
  4  BEGIN
  5    SELECT mySequence.NEXTVAL INTO :NEW.seq FROM DUAL;
  6  END myTrigger;
  7  /

Trigger created.

SQL> SHOW ERRORS
No errors.
SQL> INSERT INTO myTable (id) VALUES (12367);

1 row created.

SQL> INSERT INTO myTable (id) VALUES (8596);

1 row created.

SQL> INSERT INTO myTable (id) VALUES (989);

1 row created.

SQL> SELECT * FROM myTable
  2  /

       SEQ         ID
---------- ----------
         1      12367
         2       8596
         3        989

3 rows selected.

SQL>