How do I do ssh root? I already have working SSH for "admin" and my local user

I have a ReadyNAS with firmware version 6.10.3. I have some experience with Unix and Linux, so I have set up SSH and I log in into the NAS box as my local user (I've created a local user) and as admin.

Therefore, I can log in by doing "ssh shuvam@nasbox" (where "shuvam" is the local user) and by "ssh admin@nasbox" (where "admin" is of course the admin account. But I cannot do "ssh root@nasbox". It says "Permission denied".

When I login as "shuvam", my userID on the NAS box is 100. As "admin", my userID is 98. Just FYI.

Why can't I do "ssh root@nasbox"?

I need to login as "root" because I want to run "freshclam", which I believe does not work unless I run it as root. I am getting antivirus database update errors, and so I want to run "freshclam" manually.