As I came across this post after reaching some failures, I wanted to at least share my experience.
At this time of this writing, it appears that ssh key is limited to a simple RSA keygen.
I tried using ed25519 but that failed.
I tried using rsa -b 4096 but that also failed.
I did not even tried to make a comment as part of the ssh keygen.
Using POP!_OS 22.04 LTS, I used the following commands:
ssh-keygen -t rsa -f id_rsa-ap6pro-simple-first
If you use an .ssh/config, you can add this entry:
Host appro6
Host appro6.mydomain.tls
User root
IdentityFile ~/.ssh/id_rsa-ap6pro-simple-first
hope this helps…