If its just the last name, you needn't really be bothered about it. That will not affect your merit in any way so it is still fine. When the admit cards are released for downloading, you can reach out to them stating that the last name needs to be added. Either its added or not, as long as you have proof that it is you, ot should not stop you from appearing for the exam, attend the counselling and join a college if you get a seat.