first check ur /root/.ssh folder:

cd /root/.ssh
ls
//output like 
190709 4 -rw------- 1 root root  566 Jun 20 13:19 authorized_keys
190718 4 -rw-r--r-- 1 root root  444 Jun 28 12:48 known_hosts

The files id_rsa and id_rsa.pub is missing.
So now u can search ifor the 2 files in ur system. if u found it, give the correct path to id file on ssh-copy-id

ssh-copy-id -i /path/to/id_file -p SSH-PortNumer user@hostname

otherwise u have to create the id file:

ssh-keygen
//output
enerating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /root/.ssh/id_rsa
Your public key has been saved in /root/.ssh/id_rsa.pub
The key fingerprint is:

ssh-copy-id -p SSH-Port-Numer user@hostname