How to log in as non root user via SSH?

I just created my first Ubuntu droplet and supplied my public SSH key which is contained in ~/.ssh/id_rsa.pub on my local machine at the time I was creating my droplet. I can SSH into my machine as root just fine via the command ssh root@remote_host where remote_host is my droplet’s IP address as given in the console. However, I would like to prohibit root logon to my droplet.

I created a new user by executing the commands adduser newusername followed by gpasswd -a newusername sudo to give the new user elevated privileges. I then manually copied over the contents of my public SSH key (from my local machine) following these instructions over to newusername.

$ mkdir .ssh
$ chmod 700 .ssh
$ cd .ssh
$ vim authorized_keys

I then pasted in the public key and saved the file

$ chmod 600 authorized_keys
$ exit
# service ssh restart

When I tried to SSH in from my local machine using the command ssh newusername@remote_host, I get the message Permission denied (publickey). I even went back into the droplet and added the line AllowUsers newusername to the /etc/ssh/sshd_config file but keep getting the same error.

UPDATE: I also tried running the following commands on my local machine, which is also running Ubuntu, but this did not work either:

$ eval `ssh-agent -s`
$ ssh-add ~/.ssh/id_rsa