This question already has an answer here:
I scanned my system with CLamAV by the terminal and I wanna know what files are infected. How can I get a report for that?
I want a report on my last scan, not on the next one
Try making the log file beforehand like http://askubuntu.com/a/220100/216048 and check your write permissions (sudo it to be sure).
What error do you get?
(I would've posted a trivial comment but I don't have permissions to do that on other posts)