Mix task "release" not found error

I was setting up an Ubuntu 16.04 server for my Elixir/Phoenix application with edeliver when edeliver failed with the error message ** (Mix) The task "release" could not be found along with a exit code of 1. If I go to my build server, git pull my app and run MIX_ENV=prod mix release or just mix release after getting dependencies, I get the same error.

However, the same application works without any problem locally.

My erlang version is erts-9.0 and elixir version is 1.5.5 on both the server and my local computer. Hex version is also the same at 0.16.1.

Phoenix was installed using this command: mix archive.install https://github.com/phoenixframework/archives/raw/master/phx_new.ez.

This is my mix.exs file.

defmodule MyApp.Mixfile do
  use Mix.Project

  def project do
    [
      app: :MyApp,
      version: "0.0.1",
      elixir: "~> 1.4",
      elixirc_paths: elixirc_paths(Mix.env),
      compilers: [:phoenix, :gettext] ++ Mix.compilers,
      start_permanent: Mix.env == :prod,
      aliases: aliases(),
      deps: deps()
    ]
  end

  # Configuration for the OTP application.
  #
  # Type `mix help compile.app` for more information.
  def application do
    [
      mod: {MyApp.Application, []},
      extra_applications: [:comeonin, :logger, :runtime_tools, :earmark, :distillery, :edeliver]
    ]
  end

  # Specifies which paths to compile per environment.
  defp elixirc_paths(:test), do: ["lib", "test/support"]
  defp elixirc_paths(_),     do: ["lib"]

  # Specifies your project dependencies.
  #
  # Type `mix help deps` for examples and options.
  defp deps do
    [
      {:phoenix, "~> 1.3.0"},
      {:phoenix_pubsub, "~> 1.0"},
      {:phoenix_ecto, "~> 3.2"},
      {:mariaex, ">= 0.0.0"},
      {:phoenix_html, "~> 2.10"},
      {:comeonin, "~> 4.0"},
      {:phoenix_live_reload, "~> 1.0", only: :dev},
      {:gettext, "~> 0.11"},
      {:earmark, "~> 1.2.3"},
      {:cowboy, "~> 1.0"},
      {:edeliver, "~> 1.4.4"},
      {:distillery, "~> 1.5.1"}
    ]
  end

  # Aliases are shortcuts or tasks specific to the current project.
  # For example, to create, migrate and run the seeds file at once:
  #
  #     $ mix ecto.setup
  #
  # See the documentation for `Mix` for more info on aliases.
  defp aliases do
    [
      "ecto.setup": ["ecto.create", "ecto.migrate", "run priv/repo/seeds.exs"],
      "ecto.reset": ["ecto.drop", "ecto.setup"],
      "test": ["ecto.create --quiet", "ecto.migrate", "test"]
    ]
  end
end

My edeliver configuration file .deliver/config is as follows:

APP="MyApp"

BUILD_HOST="1.1.1.1"
BUILD_USER="ubuntu"
BUILD_AT="/home/ubuntu/app_build"

PRODUCTION_HOSTS="1.1.1.1" 
PRODUCTION_USER="ubuntu" 
DELIVER_TO="/home/ubuntu/app_release" 

pre_erlang_get_and_update_deps() {
  local _prod_secret_path="/home/ubuntu/app_config/prod.secret.exs"
  if [ "$TARGET_MIX_ENV" = "prod" ]; then
    __sync_remote "
      ln -sfn '$_prod_secret_path' '$BUILD_AT/config/prod.secret.exs'
    "
  fi
}

TLDR: point your branch explicitly and make sure, that folder /rel/ under git and committed:

$ mix edeliver build release --branch=feature/deploy

I had similar error: ** (Mix) The task "release" could not be found and when I tried to find a solution, this unresolved discussion got my attention several times. I realized, that mix task release is in the /rel/config.exs. And if you use command $ mix edeliver build release as mentioned in literally all tutorials, edeliver gets master branch version (no matter from what branch you try to build).