Cannot download dependency from github repository

I am trying to run mix des.get but the command is failing with the following error:

* Updating myapp (git@github.com:username/myapp.git)
ERROR: Repository not found.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
** (Mix) Command "git --git-dir=.git fetch --force --quiet --progress --tags" failed

What makes this frustrating is that it works in my local machine, but when I run it in the deploy machine it simply complains about permissions.

I have this dependency in my mix.exs like the following:

defp deps do
    [
      {:clint,          git:  "git@github.com:username/myapp.git",          tag: "0.1.2"  }
    ]
  end

Am I missing something?