How to connect SSH Key and fix bad configuration option ssh-ed25519?

Considering Linux and MacOS OpenSSH implemention can diff, you can follow this article

I got this error when I attempted to use the .ssh/config file from macOS on my laptop with LMDE4:

greys@xps:~ $ ssh mcfly
/home/greys/.ssh/config: line 14: Bad configuration option: usekeychain
/home/greys/.ssh/config: terminating, 1 bad configuration options

Because this option is a generic one I have at the very top of my .ssh/config file, it means I can’t connect anywhere until I fix or comment this "UseKeychain yes" option.

Luckily, there’s a simple enough fix: just add the IgnoreUnknown directive into .ssh/config right above the "UseKeychain yes", and it will be ignored on Linux systems.
On macOS it will still keep working as intended.

Here’s how the updated fragment of .ssh/config should look:

IgnoreUnknown UseKeychain
UseKeychain yes

It sounds like you've accidentally placed an SSH public key into ~/.ssh/config, since ssh-ed25519 is the first part of a certain type of public key.

If that's the case, you can edit that file and remove the offending line, which should make OpenSSH happy again.

Thanks to everyone who contributed answers. The problem is resolved. Followed git documentation step b step again.

https://docs.github.com/en/github/authenticating-to-github/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent