Unfair delay of my Studies!!!

I am registered with the Accounting Academy and I have been having slight teething problems with institution, some of which I thought I have overlooked. But this one I am really taking it very hard. I booked for exams on 29 January 2025 for 2025 April Exams. Since my tuition fees are paid in full, I just send and invoice and they take care of things. I sent an invoice and the person I sent it to acknowledged but today I learnt that the invoice had lapsed. I do not find the information or notification of the fact that the invoice had lapsed in 24hrs, I am not also finding any information that says I need to rebook by a certain time for the April exams. And now I have been shut out of the April Examinations. I feel this is unfair and a disadvantage to my studies and unnecessary delays on my studying. My please and request is to write my exam on the date that I have requested to write on, which is in the month of April 2025.