Are you running clamscan as an unprivileged user? The directory /var/lib/clamav is probably owned by the clamav user and the clamav group.
On my system (Slackware), the equivalent directory (/usr/share/clamav as it happens) has rwxrwx--- permissions, so only members of the clamav group, the clamav user and root have permissions to read/write there.
As a solution, you can either run clamscan as root or add yourself to the clamav group. There may also be more or less secure ways of gaining access to that directory (chmod...).