By default, logging in as root using password is disabled. You should change the
PermitRootLogin setting in
/etc/ssh/sshd_config accordingly to allow that, if it is what you want. Alternatively set up public key authentication for the root account. See e. g. How to set up passwordless SSH access for root user for an example. Unfortunately this will be impossible to do without help from your hoster if you really locked yourself out and logging in as john also does not work.
In addition to this, if you do not add root to the
AllowUser directive as well, the setting of PermitRootLogin will have no effect anyway.
So to fix your particular problem make those two settings in
sshd_config look like this:
AllowUsers root john
and then restart sshd or reboot the server. Afterwards you should be able to log in as root using a password.
As for the error message when you are trying to log in as john: This does not look like a normal ssh or Ubuntu error message but rather something your hoster added. You should probably do what it says and contact support.
In general, if you are messing with sshd settings there is an easy way to test things: Simply restart the sshd service using
service ssh restart and try things out from another terminal. Do not terminate your existing session. If something goes wrong you still have the running session and can revert the wrong settings.
Rebooting the whole server is never necessary for activating changes to sshd.