can't connect to github via ssh

Did you try to add github as known host?

If you have an existing known_hosts file and do not want to overwrite (as @Puce suggested in the comment), use this.

ssh-keyscan -t rsa github.com >> ~/.ssh/known_hosts

This will add the rsa key to the end of the known_hosts.

If you have no idea what known_hosts is/means, or you never engaged with it for sure, you can simply write a new one, but note that this will remove the existing one if you had any.

ssh-keyscan -t rsa github.com > ~/.ssh/known_hosts