Before creating the pento database, the book instructs,

First, make sure you have Postgres installed and running on localhost, accessible with the default username postgres and password postgres.

Some readers may have Postgres installations that require different values for username, password, etc.

I suggest mentioning the file pento/config/dev.exs, where the Postgres configuration can be changed. For example,

# Configure your database
config :pento, Pento.Repo,
  username: "postgres",
  password: "zvbxrpl",
  hostname: "localhost",
  database: "pento_dev",
  stacktrace: true,
  show_sensitive_data_on_connection_error: true,
  pool_size: 10