Most universities understand that resit candidates often don't have new predicted grades for exactly the same reason as you - they're studying on their own. These universities can use your previous results, and the knowledge that you're spending another year studying, to make a reasonable prediction themselves. (For example, they may assume that you'll go up by one grade in each subject.)
If there anything in your application which would have led this university to assume that you're using a tutor? I think your only option is to respond by saying that you're studying independently without a tutor, and are therefore not in a position to provide a formal prediction. You could provide your own prediction if you like, but I'm not sure what weight this would carry.
Which university is this, by the way?