Oracle Trigger BEFORE INSERT has No data found

SELECT NAME INTO PASSENGER_NAME FROM PASSENGER JOIN BOOKING
ON PASSENGER.PASSENGER_ID = BOOKING.PASSENGER_PASSENGER_ID
WHERE BOOKING.PASSENGER_PASSENGER_ID = :NEW.PASSENGER_PASSENGER_ID
AND ROWNUM  = 1;

SELECT NAME INTO PASSENGER_NAME 
FROM PASSENGER 
WHERE PASSENGER.PASSENGER_ID = :NEW.PASSENGER_PASSENGER_ID;

create or replace trigger tvideo_2 after insert on video
begin
  update video
  set age = 18
  where title like '%18+%'
  and age != 18;
end;
/

create or replace trigger tvideo_2 after insert on video
begin
  for rec in (
    select videoid, title
    from video
    where title like '%18+%'
    and age != 18
  )
  loop
    update video
    set age = 18
    where video.videoid = rec.videoid;
    dbms_output.put_line('Video '||rec.videoid||' has been updated age to 18+');
  end loop;
end;
/

create or replace trigger tvideo_2 before insert on video for each row
begin
  if :new.title like '%18+%' then
    :new.age := 18;
    dbms_output.put_line('Video '||:new.videoid||' has been updated age to 18+');
  else
    dbms_output.put_line('Video '||:new.videoid||' is not 18+!');
  end if;
end;
/

CREATE OR REPLACE TRIGGER trg_reserve_products
BEFORE INSERT ON reserve_products
FOR EACH ROW
DECLARE
  v_count NUMBER;
  v_p_id NUMBER(6);
  v_c_id NUMBER(6);
  v_p_quantity products.p_quantity%TYPE;
  v_p_name products.p_name%TYPE;
BEGIN
  SELECT COUNT(*)
  INTO v_count
  FROM products
  WHERE p_id = :NEW.p_id;
  IF v_count = 0 THEN
    raise_application_error(-20000 , 'Invalid Product ID.');
  END IF;

  SELECT COUNT(*)
  INTO v_count
  FROM customers
  WHERE c_id = :NEW.c_id;
  IF v_count IS NULL THEN
    raise_application_error(-20000 , 'Invalid Customer ID.');
  END IF; 
  
  SELECT p_quantity, p_name
  INTO v_p_quantity, v_p_name
  FROM products
  WHERE p_id = :NEW.p_id;  
  IF v_p_quantity < 1 THEN
    raise_application_error(-20000 , v_p_name || ' 4 is currently out of stock.');
  END IF; 
  
  UPDATE products
  SET p_quantity = p_quantity - 1
  WHERE p_id = :NEW.p_id;
END trg_reserve_products;

SELECT * FROM products;

INSERT INTO reserve_products VALUES(1, 101, 1);
INSERT INTO reserve_products VALUES(2, 46, 1);
INSERT INTO reserve_products VALUES(3, 46, 2);
INSERT INTO reserve_products VALUES(4, 46, 3);

SELECT * FROM products;

SELECT * FROM reserve_products;

create or replace trigger t
  before insert on vol 
  for each row          --> edited
declare 
  etat VARCHAR(10);
BEGIN 
  -- MAX will prevent NO-DATA-FOUND 
  -- Also, you don't need join - use :NEW.ID_AV which is equal to currently inserted value
  select max(avion.etat)
  into etat 
  from avion
  where avion.id_av = :new.id_av;

  -- NVL because - if SELECT returns, nothing, you can't compare NULL with 'disponible'
  IF nvl(etat, 'x') <> 'disponible'
  THEN 
    RAISE_APPLICATION_ERROR( -20001, 'insertion imposible');
  END IF;
END t;
/ 

 SQL> CREATE TABLE test(
                  id       number generated always as identity primary key,
                  item     varchar2(100),
                  quantity int
 );
 /
 SQL> INSERT INTO test(item,quantity) VALUES ('KA1',5);
 SQL> INSERT INTO test(item,quantity) VALUES ('KA2',2);

 SQL> CREATE OR REPLACE PROCEDURE XXI_MULTI_PR_REMOVE( I_ITEM varchar2 ) IS
 BEGIN
  DELETE TEST WHERE ITEM = I_ITEM;
 END;
 /
 SQL> CREATE OR REPLACE TRIGGER MERG_DUP
 BEFORE INSERT ON TEST
 FOR EACH ROW
 DECLARE  
      v_qty NUMBER;
 BEGIN
    BEGIN
      SELECT SUM(NVL(QUANTITY,0)) INTO v_qty FROM TEST WHERE ITEM = :NEW.ITEM;
     EXCEPTION WHEN OTHERS THEN v_qty := NULL; 
    END;  
    IF ( v_qty IS NOT NULL ) THEN
        XXI_MULTI_PR_REMOVE(:NEW.ITEM);
       :NEW.QUANTITY:=:NEW.QUANTITY+v_qty;    
    END IF;
 END MERG_DUP;
 /
 SQL> INSERT INTO test(item,quantity) VALUES ('KA3',6);
 SQL> COMMIT;
 SQL> SELECT * FROM test;