"Illustrator tried but could not save the file in AI format"

I downloaded Illustrator to my Macbook Pro for an online graphic design class. My teacher requires me to submit my work in AI format. Every time I go to save my work in AI format I get a pop up window saying "Illustrator tried but could not save the file, click learn more to try alternate options for saving your file". I have been saving my files and submitting them in EPS format, but I have been losing significant points on my assignments. I am not a computer expert so I have no idea what to do to resolve the issue. I have done much online research but no solution has presented itself. It has nothing to do with the fonts I am using because I cannot even save a blank document in AI format. Any ideas?