"This account is currently not available" error when trying to ssh

While I agree with the others that allowing login through SSH through the www-data user is generally a bad idea, once you've logged in with a normal user it may be useful to run multiple commands concurrently with the permissions set of the www-data user. In that case, one can run

sudo su -l www-data -s /bin/bash

and you will be able to access your files as the www-data user.

You're getting the This account is currently not available. error because the shell for the user www-data is set to /usr/sbin/nologin, and it's set for a very good reason. You should not log in as www-data, it's a special user/group used by the web server, not intended for regular shell use.

EDIT: It is an especially bad idea to give sudo rights to www-data. If Apache was intended to run with root permissions, it wouldn't have it's own group. By doing this, you are creating huge security holes. You have been warned.