Drush Aliases into Remote Hosts not Working - Tried everything
Try this first:

ssh-copy-id username@url.com


Make sure that the username and url match what is in your alias, of course.  The "Permission denied" error is complaining that the remote site does not have your public key stored in its "authorized keys" file.  ssh-copy-id will fix this for you.

If you are on a Mac, you can get ssh-copy-id from here:

https://github.com/beautifulcode/ssh-copy-id-for-OSX

It should be bundled with openssh on linux.