How to execute AFTER INSERT TRIGGER with an auto-incremented (seq.NEXTVAL) column value in Oracle PL/SQL?
The trigger you have written will try and insert a second row, and is causing the triggers view of the table to mutate (as it will end up in an infinite loop, essentially).

You actually only need a BEFORE INSERT trigger that sets :NEW.Airline_No : 

create trigger new_airline_btrig 
  before insert on airline
  for each row
begin
  :new.airline_id := airline_seq.NEXTVAL;
end;
/


You can then insert using just:

INSERT INTO Airline (Airline_Name, Country)
   VALUES ('Malaysia Airlines', 'Malaysia')


:new.columnname alters the given inserted value (in this case, from NULL to the sequence value, as airline_id was not provided).