Comment 3 for bug 1718112

Thank you for taking the time to file a bug report.

It looks like the first issue is that the clamav user already exists.
This may have been from a previous failed install, or from manually
attempting to add the user. The user should not be present though before
installing the package.

Second, you mention trying to install in different ways. You should
always default to using apt when installing packages. My suggestion
would be to get rid of the existing user and try the following:

apt purge clamav
[remove the user if it still exists]
apt update
apt upgrade
apt install clamav

Since it seems likely to me that this is a local configuration problem,
rather than a bug in Ubuntu, I am marking this bug as 'Incomplete'.

However, if you believe that this is really a bug in Ubuntu, then we would
be grateful if you would provide a more complete description of the problem
with steps to reproduce, explain why you believe this is a bug in Ubuntu
rather than a problem specific to your system, and then change the bug
status back to "New".

For local configuration issues, you can find assistance here:
http://www.ubuntu.com/support/community