Need to create a trigger that updates without adding same number - oracle

Question

I need to create a trigger that activates after i make an update in Table A, registering in an audit log the number that i updated in Table A, but if the number has already been added (example the trigger tries to add 1 when there is a 1 already)it must ignore it and only let the first one.
Example:
Table A updates with 5,5,6,8,4,4
Then the audit log must save 5,6,8,4
The trigger i have already:
CREATE OR REPLACE TRIGGER registro_aeropuerto
AFTER UPDATE ON AEROPUERTO
FOR EACH ROW
DECLARE
A INT;
B INT;
BEGIN
A := table_A_updated_column_value;
SELECT CASE
WHEN EXISTS(SELECT * FROM Audit_log WHERE A = Coordinator)
THEN 1
ELSE 0
END INTO B FROM DUAL;
IF B = 0
THEN
INSERT INTO Audit_log(Coodinator, Date) VALUES (A, trunc(sysdate));
END;
Whenever i try to execute the trigger it gives me the next error:
The symbol ";" has been found when the it was expected:

Answer 1

Sample tables (upd_col_value is column you're updating; you named it "table_A_updated_column_value")
SQL> create table aeropuerto (upd_col_value number);
Table created.
SQL> create table audit_log (coordinator number, datum date);
Table created.
Trigger can be simplified; no need to declare any additional variables nor to check first and insert next; do it in the same select statement:
SQL> create or replace trigger registro_aeropuerto
2 after update on aeropuerto
3 for each row
4 begin
5 insert into audit_log (coordinator, datum)
6 select :new.upd_col_value, sysdate
7 from dual
8 where not exists (select null
9 from audit_log a
10 where a.coordinator = :new.upd_col_value
11 );
12 end;
13 /
Trigger created.
Testing:
SQL> insert into aeropuerto (upd_col_value) values (1);
1 row created.
SQL> select * from audit_log;
no rows selected
There's nothing in the log because nothing was updated. So, let's update it:
SQL> update aeropuerto set upd_col_value = 5;
1 row updated.
SQL> select * from audit_log;
COORDINATOR DATUM
----------- -------------------
5 15.09.2021 07:14:46
SQL>
OK; log now contains a row. Another update:
SQL> update aeropuerto set upd_col_value = 6;
1 row updated.
SQL> select * from audit_log;
COORDINATOR DATUM
----------- -------------------
5 15.09.2021 07:14:46
6 15.09.2021 07:15:37
SQL>
Right; two rows, as 5 was updated to 6. What happens if we update 6 back to 5?
SQL> update aeropuerto set upd_col_value = 5;
1 row updated.
SQL> select * from audit_log;
COORDINATOR DATUM
----------- -------------------
5 15.09.2021 07:14:46
6 15.09.2021 07:15:37
SQL>
Nothing happened; row with coordinator = 5 was in the table already so new row wasn't added.