I miss the line

...
config :account_service, ecto_repos: [AccountService.Repo]
...

in your config/config.ex file.

Ensure the below and give it a re-try.

STEP 1. Define the dependencies required, mix.exs

defp deps do
    [
      {:ecto_sql, "~> 3.1"},
      {:postgrex, "~> 0.14.3"}
    ]
 end

STEP 2. Get the dependencies defined above

mix deps.get

STEP 3. Configure the Repo the application

config :helloapp, ecto_repos: [HelloApp.Repo]

Note: the :helloapp must be match the app name defined in the application.ex file. In my case I have created the application using the below command

mix new hello app --app hello app --sup

STEP 4. Configure the the Repo itself defined above

config :helloapp, HelloApp.Repo,
      host: "localhost",
      port: "5432",
      database: "helloappdb",
      username: "postgres",
      password: "postgres"

STEP 4. Define the HelloApp.Repo module in repo.ex file in /lib/{app name}

defmoudle HelloApp.Repo do
use Ecto.Repo,
    otp_app: :helloapp,
    adapter: Ecto.Adapters.Postgres
end

STEP 5. Now, lets create the database

mix ecto.create

The output should be as below.

Compiling 1 file (.ex)
Generated helloapp app
The database for HelloApp.Repo has already been created.

Hope it helps somebody.

Are you sure your app’s atom name is :MyApp and not :myapp?

Can you also show your MyAppRepo.ex file?