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I'm add tcn in table:

alter table accounts add TCN integer;

And UPDATE accounts set TCN = DBMS_UTILITY.GET_TIME;

And tcn can not be null alter table accounts modify TCN not null;

I created trigger before update

SQL> create or replace trigger acc_preupdate
 2 before update on accounts
 3 for each row
 4 begin
 5 if(:NEW.TCN != :OLD.TCN+1) THEN
 6 RAISE_APPLICATION_ERROR(-20000, 'Concurrency Failure');
 7 end if;
 8 :NEW.TCN := DBMS_UTILITY.GET_TIME;
 9 END;
 10 /

And trigger before insert

SQL> create or replace trigger acc_preinsert
 2 before insert on accounts
 3 for each row
 4 begin
 5 :NEW.TCN := DBMS_UTILITY.GET_TIME;
 6 end;
 7 /

I insert in table

SQL> insert into accounts (acc_id, acc_name, acc_amount, acc_date)
 2 values(acc_seq.nextval, 'petar', 15000, sysdate);

And when I want to update the table, I get this error

SQL> update accounts
 2 set acc_amount = 1000000
 3 where acc_id = 1;
update accounts
 *
ERROR at line 1:
ORA-20000: Concurrency Failure
ORA-06512: at "PETAR1.ACC_PREUPDATE", line 3
ORA-04088: error during execution of trigger 'PETAR1.ACC_PREUPDATE'

now I select users with id 1.

select acc_name, acc_amount from accounts where acc_id = 1;

And when I try to update the table again, I get the same error.

Why can not I update the table?

I use Oracle 12c r2

In your final update statement you do not specify an update to TCN, so in the update trigger, the value of :OLD.TCN is what you had inserted before (via the insert trigger). Since the update does not explicitly change it, also :NEW.TCN is that same value.

So :OLD.TCN is equal to :NEW.TCN and making the if condition in the update trigger true, raising your custom exception.