can't access root using ssh Permission denied (publickey)

I going through the tutorial for initial server setup https://www.digitalocean.com/community/tutorials/initial-server-setup-with-ubuntu-14-04… At step 4 I need to manually install the key, as it said in topic Add Public Key to New Remote User I need add the public key to a special file in the user’s home directory, but when trying to access root again from my laptop terminal using ssh root@my_ip_address_domain it showing error Permission denied (publickey), I’m using ubuntu 16.04 and created droplets using ssh… Sorry if this question is simple, this is my first time using server