www account is usually used for web server processes and direct login to shell is prohibited for security reasons. It is imperative it stays that way to prevent a hacking attempt on your website to get elevated rights on the server. just run
grep www /etc/passwd
and note the shell assigned to the account. It must be something like /bin/nologin or something to this sense. If you have root rights to this server, you can modify it but, I strongly suggest not to do such a thing. If you are just testing something, do it quickly and revert it back.
Of course this suggestion stands if you are running a public facing website on this server.