Could you please make a small repo on github with the code to reproduce this? That would help us identify the problem on our own machines and then walk you through a (hopefully) solution.

What does your mixfiles application function look like?

Is it (close to) the following?

def application do
  [mod: {Friends.Application, []}]
end

As @NobbZ suggested, you need to add your application to your application function in mix.exs (https://github.com/dersar00/friends/blob/master/mix.exs#L15-L20), otherwise it won’t be started.

Try replacing

  def application do
    [
      # NOTE: I think you should use either :extra_applications or :applications
      # see https://www.amberbit.com/blog/2017/9/22/elixir-applications-vs-extra_applications-guide/
      applications: [:method_missing, :logger, :mongodb_ecto, :ecto],
      extra_applications: [:logger]
    ]
  end

with

def application do
  [
    mod: {Friends.Application, []}, # points to the module with application
    extra_applications: [:logger]
  ]
end

What version of ecto are you using? It might be outdated.

Try replacing

  def start(_type, _args) do
    # List all child processes to be supervised
    children = [
      Friends.Repo
      # {Friends.Worker, arg},
    ]

with

def start(_type, _args) do
  import Supervisor.Spec

  # List all child processes to be supervised
  children = [
    supervisor(Friends.Repo, [])
    # {Friends.Worker, arg},
  ]

in https://github.com/dersar00/friends/blob/master/lib/friends/application.ex#L11