Sometimes the council puts and incorrect verification code on the letter, and when this happens the tribunal will change it. Your aunt will need to call them up and ask for the verification code.
Another scenario might be that if she appealed by sending a paper form in the post, the verification code might not have been set up, in which case you can just ask them to put it in.
Don't put the verification code on here or anyone on the internet will be able to get your Aunt's postal address, phone number etc as that's all recorded on the case record.