I really need help from someone who's willing to overlook (and correct) a work I made for university. There is a bug in there but I just can't figure out what I did wrong

I have to submit my work tomorrow by noon and I've just about everything I know. I haven't got anyone else to ask for help

As soon as I hit the "Opereeri" button (as seen here) I get the following error (as seen here)

http://www.ramones.pri.ee/arvuti2.xls - here is the file itself.