Unable to graduate because of unprofessionalism

I was advised that I'm left with one subject to complete my diploma, I wrote the subject I'm may and passed. I was so happy that finally after four years I can finally graduate. I was informed that they made a mistake and I'm still left with one more subject to finish my diploma, I was so angry and the worst part is that I asked them to write this remaining subject in October but they refused and told me the registration for October is closed. Now I have to wait for another year to graduate. Student number 145984