Yes, you can still serve an N5 for damage in this situation. It can be given where you can show that the tenant wilfully or negligently did the damage. If they are claiming it is ordinary “wear and tear”, you will have to be prepared to show how this is not the case.
If you have a move-in inspection report or pictures of before they moved in, this will be relatively easy. If you do not, then it is your word against theirs.
Keep copies of correspondence between you, and copies of your notices to enter in case they are required at a hearing regarding this matter in the future.