ETA: If you install of Start9 OS is not on your main system but on a headless server, after installing Start9 OS, log into that server from a web broswer. Copy the key file from terminal (if using linux) and paste into the key file box. It should work.
I followed the guide to setup an SSH key file on my linux machine for my new Start9 server using Start9 | Using SSH
Using cat ~/.ssh/id_ed25519.pub on terminal, I was showed a key string
According the guide posted above, I “pasted” that into the SSH key and verified that everything matched but no matter what I do, all I see is an RPC ERROR: Invalid Request key format.
I followed the steps in the guide again and I created a new SSH keyfile as displayed in the attached picture. After pasting that into the SSH key file space, the same errors shows.
I feel dumb. I use Start9 as a headless server. I forgot that I could log into it from my browser. I copied the key file and added to the keyfile, it worked right away.
Step 1: Log into your Start9 OS (if using as a headless server on your network)
Step2: Copy the key from terminal (or whatever distro you are using)
Step 3: Paste that into the SSH key file. It should work.