ssh - How to solve "sign_and_send_pubkey: signing failed: agent refused operation"?

Question

Configuring a new Digital Ocean droplet with SSH keys. When I run ssh-copy-id this is what I get:

ssh-copy-id user@012.345.67.89
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
sign_and_send_pubkey: signing failed: agent refused operation
user@012.345.67.89's password: 

Number of key(s) added: 1

Now try logging into the machine, with:   "ssh 'user@012.345.67.89'"
and check to make sure that only the key(s) you wanted were added.

However, when I then attempt to ssh in, this happens:

ssh user@012.345.67.89
sign_and_send_pubkey: signing failed: agent refused operation
user@012.345.67.89's password: 

Upon entering the password, I am logged in just fine, but this of course defeats the purpose of creating the SSH key in the first place. I decided to take a look at the ssh-agent server-side and here's what I get:

user@012.345.67.89:~# eval `ssh-agent -s`
Agent pid 5715
user@012.345.67.89:~# ssh-add -l
The agent has no identities.

user/.ssh/authorized_keys does contain an ssh-rsa key entry, as well, but find -name "keynamehere" returns nothing.

Answer 1

Run ssh-add on the client machine, that will add the SSH key to the agent.

Confirm with ssh-add -l (again on the client) that it was indeed added.