problems applying ssh keys with Ubuntu 18

Right, so before you send the link I have been following this step-by-step https://www.digitalocean.com/community/tutorials/initial-server-setup-with-ubuntu-16-04#step-four-

I got to the section where it says

Now you will be in your new user’s home directory.

Create a new directory called .ssh and restrict its permissions with the following commands:

mkdir ~/.ssh
chmod 700 ~/.ssh
Now open a file in .ssh called authorized_keys with a text editor. We will use nano to edit the file:
$ nano ~/.ssh/authorized_keys

however I don’t have that file name in that directory so I created it and pasted my pub ssh key in there as it says, and then followed the tutorial to the end and where it says that I could sign in I tried to and I still get the error: christian@<ip_addess>: Permission denied (publickey).

but then I noticed it was Ubuntu 16, and I have 18. So I switched to the docs about Ubuntu 18 and there isn’t anything about setting the key, just the firewall. So I am kinda in limbo as to what I can do

https://www.digitalocean.com/community/tutorials/initial-server-setup-with-ubuntu-18-04

at step 5 it says that if password is enabled (which is it) then I can sign in with christian@<ip_address> which I can’t because I get the error stated above

I can do: $ ssh root@<ip_address> then enter a password but I cannot do $ ssh christian@<ip_address> because of Permission denied(publickey). So I don’t know how to resolve this.

thank you!