postgresql throw an error if were concurrent inserts

I have table account

create table account (
    user_id int primary key,
    acc int,
    amt int
);

with the next data

 user_id | acc  | amt 
---------+------+-----
     727 | 2620 |  10
     777 | 2620 | 100

and I want to insert new row based on existing with conreate acc, and if there would be concurrent insert with this acc I want to rollback transaction

I tried to use serializable isolation level:

T1

begin;
set transaction isolation level serializable;

insert into account (user_id, acc, amt) 
values (-1, 2630, (select sum(sub.amt) 
                   from account sub where sub.acc = 2620));

-- here T2 made an insert, but i didn't get expected serialization error
commit;

T2

insert into account(user_id, acc, amt) 
values(737, 2620, 100);

and as result there was no conflict, but should be. What am I missing?